Varsovian:
Why? This game is scary - when it cames to the amount of rules. It crunchier than GURPS - the corerulebook has nearly 600 pages. And then, there are bestiaries, player and GM guides, advanced classes, books like Ultimate Magic / Campaign / whatever... I'm seriously overwhelmed.
By no means does anyone have to be aware of every single rule, choice and option in a D&D game. That's one of the nice things about level-based games: everyone only really needs to be familiar with the things relevant to the level at which they're playing. That's going to be a fair amount even at first level, since there can be a lot of basic rules, but it's still just a segment of what's necessary.
Then there's stuff that just isn't going to come up in your games. If you're not planning to involve seafaring, don't bother learning about the relevant rules, feats, items or monsters. Etc. That's one of the nice things about dungeon-based adventures.
Varsovian:
And of course, before starting to GM, one needs to come up with a setting. That means decisions - on style and mood, on races available, on languages, on gods... And when you come up with gods, you need to create a whole cosmology for them... Again, more complicated than GURPS!
How to even start with this game??? Ugh...
The great thing is that you don't need to make all those choices yourself. Get some players interested in the general idea of a simple Pathfinder game, and then collaborate with them on the details.
And contain those details. Why do you need a cosmology at all? Not every game depends on that level of detail.
I can understand
wanting those details, or feeling that they're necessary, but games really can get by on less. One thing they can't get by on is a frustrated, overwhelmed GM, so you'll need to find a balance. Remember that it's fiction, and don't hold yourself to too high a standard, as most of them are unrealistic anyway. You're not Tolkien, and you don't have his kind of time, or his goals. You're just playing a game. Let your guide be more modern stories where lots the details aren't written yet, because the writer (like most GMs) doesn't know how long the story will need to last. My bet would be that a lot of the stories you enjoy didn't initially have nearly the backstory they appeared to have down the road.