Why can't they just remove keystones?

Because they do not want to. Does not seem that hard to me.

It would leave a lot of champions in awkward positions of power.

Again.

For the unth time in a row.

why can't we remove items from the game? is it that hard?

Key stone was introduced for easier balancing of a class. Prior to that they could only do so by changing items shared by different classes, now they can target some class more directly.

Plus it gives a little bit more of flavor in the planning phase...

What do you find about the keystone that is annoying?

Because the hardest the game to balance, the funniest.