I am completely ignorant of LitRPG. It is entirely possible that some reader get pleasure from the insight into these numerical statistics alongside with the plot. The OP question suggests however that most author get tired of providing these numbers. More than a convention it sounds like a sign of maturation in style.

The issue: Don't tell, show it

In my opinion it is mostly an issue of showing vs. telling. You can tell the reader that the character just lost 8 points, or you can show it by letting the reader feel what the character feels. The first may be interesting in an instruction manual, the latter is more suitable for a work of narrative. I have the impression that as the writer progresses with the LitRPG they try to create a work of narrative and hope that the readers forget they have been reading an instruction manual until a few pages earlier.

The second issue: Clickbait into an uninteresting story

Crunch in the first few pages some elements that you believe would attract readers. A reader that is into RPGs may find the reading facilitated if it resembles notes from a gaming session. Unless LitRPG is intended to provide guidelines on how to play a particular RPG, this would suggest that the story is not that interesting to start with, or that it started too early in the timeline.

The awkward points

1. Why not give all the numerical data of the world?

Why limit the numerical representation to characters statistics? Why not providing the change in geographical coordinates for each movement? Or the change in temperature perceived by the character? If the counter-argument is that the numbers should just reflect a particular RPG system, why not switching for a more comprehensive one? Afterall, if someone enjoys all the number-crunching, they may enjoy even more the fact that the entire world is designed with a rigorous numerical mechanics.

2. Filler fluff

In contrast to an actual RPG session in which a character may be defeated and die as a consequence of these numerical contraptions, in a novel there is the concept of plot armour. It is quite unlikely that any major character would die as easily as the fodder that is thrown at them. This is particularly true if you have one MC. In this paradigm, all the numerical evidence about numbers and classes become filler faff as they serve no other purpose than to fill pages: the MC is going to survive and no amount of damage or class growth is going to alter the arc of the story. One could argue that the arc is determined by these numbers, and while that is true for a live session, it is less true for a novel in which the author hopes to engage readers for many and many pages.

3. The tune of ads

Using a particular RPG system and making it obvious in a commercial product is likely motivated by an advertisement campaign. The company selling the action figures, the books, and the RPG game are trying to create an easily identifiable brand. Why would you as an unrelated author imitate it? It is as if the employees of a cleaning company would sing the tune from the detergent ads every time they mop office floors.

I am completely ignorant of LitRPG. It is entirely possible that some reader get pleasure from the insight into these numerical statistics alongside with the plot. The OP question suggests however that most author get tired of providing these numbers. More than a convention it sounds like a sign of maturation in style.

The issue: Don't tell, show it

In my opinion it is mostly an issue of showing vs. telling. You can tell the reader that the character just lost 8 points, or you can show it by letting the reader feel what the character feels. The first may be interesting in an instruction manual, the latter is more suitable for a work of narrative. I have the impression that as the writer progresses with the LitRPG they try to create a work of narrative and hope that the readers forget they have been reading an instruction manual until a few pages earlier.

The second issue: Clickbait into an uninteresting story

Crunch in the first few pages some elements that you believe would attract readers. A reader that is into RPGs may find the reading facilitated if it resembles notes from a gaming session. Unless LitRPG is intended to provide guidelines on how to play a particular RPG, this would suggest that the story is not that interesting to start with, or that it started too early in the timeline.

The awkward points

1. Why not give all the numerical data of the world?

Why limit the numerical representation to characters statistics? Why not providing the change in geographical coordinates for each movement? Or the change in temperature perceived by the character? If the counter-argument is that the numbers should just reflect a particular RPG system, why not switching for a more comprehensive one? Afterall, if someone enjoys all the number-crunching, they may enjoy even more the fact that the entire world is designed with a rigorous numerical mechanics.

2. Filler fluff

In contrast to an actual RPG session in which a character may be defeated and die as a consequence of these numerical contraptions, in a novel there is the concept of plot armour. It is quite unlikely that any major character would die as easily as the fodder that is thrown at them. This is particularly true if you have one MC. In this paradigm, all the numerical evidence about numbers and classes become filler faff as they serve no other purpose than to fill pages: the MC is going to survive and no amount of damage or class growth is going to alter the arc of the story. One could argue that the arc is determined by these numbers, and while that is true for a live session, it is less true for a novel in which the author hopes to engage readers for many and many pages.

3. The tune of ads

Using a particular RPG system and making it obvious in a commercial product is likely motivated by an advertisement campaign. The company selling the action figures, the books, and the RPG game are trying to create an easily identifiable brand. Why would you as an unrelated author imitate it? It is as if the employees of a cleaning company would sing the tune from the detergent ads every time they mop office floors.