Because writing is subjective.
Some people might say that 'bestsellers' are often trashy and terrible writing and that highbrow 'literary' work is better.
But why is the opinion of a few necessarily more valid than the opinion of the masses?
Perhaps it is, if they have studied a subject and are experts. But if reading is about pleasure, then isn't everyone an expert in their own right?
The point I'm trying to make is that there is no objective right or wrong in writing, so it shouldn't be surprising that people will give different opinions.
However, to address your second point, which is, how to proceed in the face of such conflicting advice...
The answer is that you have to learn the craft yourself from your own experience and self- education. So, when faced with two different pieces of advice, try them both, and see what works for you.
Personally, my first draft is always extremely ropey, and I don't expect to keep a single word of it by the time I've finished. Others take more care with their first drafts and require less rewriting. Neither of us is wrong.
Likewise, Stephen King said thesauruses are for losers (I'm paraphrasing) but I strongly disagree with that. Sure if you use a thesaurus to increase the complexity of your writing, and use words you don't really understand that you find there, then that's problematic. But I use the thesaurus to find a word that's already on the tip of my tongue, and it serves me well.
Another common piece of advice given to beginners is 'show don't tell'. This is a great piece of advice, but that doesn't mean you have to be completely dogamtic about it. The point is to know exactly why you are using telling when you are, rather than doing it without realising.
Finally, the same for adverbs, the other dirty word in newbies' advice. Of course you can use adverbs, but if you use them badly (!) your writing will stand out as amateur.
In summary - see what works for you and don't be dogmatic about it.