Because it's damn awesome, that's why.
The elemental rock < Paper < Scissors is fun enough as it is, but then when you start playing it, you compare stats, perhaps you want a less powerful special move because the pokemon's physical attack stat isn't high enough. But then this other move has a same type attack bonus. But then this one has a chance of confusion.
And then there's abilities. Speed improves every turn? That's pretty cool, excpet your defense is awful, crap. Don't worry though, just create a substitute and protect to avoid attacks.
Then there's EV training and IVs which I've never even got in to.
And that's just the combat system.
The designs of pokemon are... interesting. There's some really rad ones and I think even the bad ones add charm to the series. For example, eggescutor is the stupidest looking thing in the world, but it suits the universe and so doesn't break immersion or ruin the game.
And shiney pokemon and legendary pokemon and different evolution conditions and certain pokemon only learn certain moves if they're inherited from their parent pokemon.
And then finally, I am a massive etymology nerd. So the wordplay and the puns in every single pokemon's name is just oh so wonderful.