Yes, I certainly wonder sometimes. I guess it's mostly to make pokémon somewhat similar in strengths and weaknesses in the game. But if we are to move outside of that, I'd agree with Normal type being a sort of jack of all trades, knowing a little of everything but not having any STAB for them.
As for Toxic and other poison moves, maybe pokémon can develop poison somewhere inside their body and with a bit of practice or the help of a TM can learn to use it in an attack? Pokémon can change shape and typing radically in evolution, why wouldn't they be able to grow subtle electric sacks in their cheeks to learn some new move?
Some pokémon are born with strange Egg moves - that's obviously a mutation depending on parents and whatnot. But maybe HMs and TMs actually work by inducing some kind of growth inside the body or an additional gland or something that allows a pokémon to physically perform a move? Not impossible :3
Another take on the whole thing is that pokémon who can learn strange attacks and seem to lack the necessary body parts or something to actually do it, they rely on Aura or the kind of power that pokémon get their strength from in the first place. They somehow conjure the attack up from pure energy and turns it elemental.
That's only for special moves though. Physical moves, I don't know :x Energy-wings lifting Doduo up into the air?
