The entire point of the Pokemon franchise is to become a Pokemon Master. The problem is, we still don't know what that means.
The creators of Pokemon Red & Green had no idea that these games would form the foundation of the biggest media franchise of all time. This is why the original generation games are so bizarre, which led to designs being changed over time (like the controversial Jynx) and story elements being censored (like the drunk guy being changed to someone who needs their coffee). The developers had no idea that that franchise was going to last this long. They even wanted Pokemon Gold & Silver to be the final entries in the series, before retooling them.
The developers had no idea Pokemon was going to last as long as it has, which is why several mysteries established in the early days of the franchise have gone unanswered. We still don't know who Ash Ketchum's father is after all of this time. This could be the reason why we don't know what it takes to become a Pokemon Master, even though it's the whole point of the series.
Bulbapedia once reached out to Pokemon.com about this mystery, and it was told that the title was meant to be open to the interpretation of the player. One idea is that anyone who completes the Pokedex and becomes the Champion would earn the title of Pokemon Master, but the anime/games never outright say this. Ash finally conquered a Pokemon League in Alola, and he still doesn't consider him a Pokemon Master, so it's clear that there is more to it.
The real reason why we don't know the details about becoming a Pokemon Master is so that the franchise can keep going. The Pokemon Company is free to keep moving the goalposts, as establishing what a Pokemon Master is creates an end goal for the series. The Pokemon video games aren't likely to end soon, so the ambiguous nature of the Pokemon Master title means that players are free to keep pursuing it for years to come.
There's a lot of great stories about Spider-Man the MCU could pull from, but you can't tell them until he's older.