This tends to be a trend in films and movies, especially in the former and especially if it's the first installment in the franchise for a hero (Think of the MCU films which, when the line first started, most of their heroes were not well known to the general public. Prior to 2008's Iron Man, the term could refer to Cal Ripkin Jr. or the Black Sabbath Song in the general public conscious. A decade later, and it's the dude with the robot armor).
This can be for multiple reasons. Stan Lee explicitly said he develop the concept of mutants in Marvel so he could be lazy and not think of origin stories for super-powers (and focus instead on the personal stories, which was a strength of both the heroes and the villains in the Franchise). It's also not uncommon for a hero to have a Dark Version of him or herself, because it allows writers to see what the hero would be like with no morals. Hence Venom to Spider-Man, Iron Monger to Iron Man, Red Skull to Captain America.
The reason its common in film and televison adaptations and original stories is because it allows for narrowing of the story. If the device that gave the hero his powers also empowers the villain, it contains the story a bit more and doesn't overly confuse the audience. Especially if the hero is not well known A-Listers and has to be introduced to a general public not familiar with comic books. For example, Iron Monger was in the first Iron Man film because a guy making another super-suit after seeing Iron Man's specs requires less explanation in the film than explaining Tony's origins and the Origin's of one of his more well known rogues gallery members (The Mandarin, who is a long hinted character in the Marvel Films that has yet to make himself fully known). Similarly, Batman Begins used Scarecrow because a big theme the shared between the hero and villain was using fear as an ally. One notable exception is the Toby McGuire Spiderman film, where Venom was held to the third instalment in favor of Spider-man's archfoes in the first and second who has very little in common in origin but are very famous villains in the franchise and instantly recognizable (Doctor Octopus was widely considered to be Spider-man's biggest villain until Spidey and Goblin had that famous encounter at the George Washington Bridge.).
Man of Steel used Zod because of the rehash of Superman's origin story. In other cases, the stories are condensed to better fit. For example, the films would show you that it was Magneto who made Xavier wheelchair bound, despite the comic depiction of the event was caused by an alien Xavier fought before he formed the X-Men. Where the two are paired off is that they are both Mutant Rights Activists but with different philosophies (Often Xavier v. Magneto is comparable to Martin Luthor King JR. v. Malcolm X, respectively, though their creation was hardly intentionally trying to link the two to the similar philosophies).
In series such as TV and the MCU, tying all heroes to a single origin is more to condense different elements into a more focused narrative. Marvel contains magic and science elements, but Magical elements in it's initial period was enschewed for weirdo objects that have rules, but not understood ones (Thor for example, is specifically an alien but does realize magic exists). Other elements got repurposed or given dual hats with similar objects to save time and make a more concise story. For example, 4 of the five reality stones in the MCU are combined with other Marvel artifacts of similar to keep the story simpilar for the film. The Space Stone is also the Cosmic Cube, the Reality Stone doubles as the Aether (similar items), The Mind Stone is Visions Sun Crystal (not to mention the origin of Pietro and Wanda's powers, since they couldn't have mutants due to liscensing issues) and the Time Stone doubles as the Eye of Agamotto. None of these artifacts had any connections in the comics, but are connected now because it's easier to attribute the items to their very powerful items that were needed to tell the story).