It might seem that it would be convenient, but not really. Imagine having a hundred or more choices to look through every time you created a new block. Or if you wanted just a tiny little tweak of the code, you'd have to go into the files themselves instead of just editing the code for the block.
As it is, you can pick and choose which blocks you want and tweak them to your heart's content. Or ask for tweaking help. That's what we're here for.
Frankly, I'd be in favor of fewer built-in blocks instead of more. But that's just me.
As I understand it, Block is writing his own gallery code.