It's a really hard problem. It costs over $100m to build and launch a spacecraft. Then the spacecraft has to catch up to the piece of junk, then do something to deal with the junk. We don't know what exactly it would do to deal with the junk.

Then the spacecraft would have to catch up to another piece of junk. That's hard because it's expensive in terms of fuel to change orbit. Thus the spacecraft would only be able to deal with a very limited number of pieces of junk before it ran out of fuel and became junk itself.

So, it would cost $millions per piece of junk. With hundreds of thousands now and millions in a Kessler scenarios it would be fiendishly expensive.

Maybe, but it'd be way cheaper to prevent it.