Note that there are loads of things that you can't keep working on after you retire. If your company had you in a position where you ran really interesting projects, all of those are the company's projects, not yours. You got to work on them because you worked there, and once you retire you can't just keep working on them, you can't even do the same work on your own because your contract made that pretty clear: now you're committing IP theft.
So you retire, and by law you're no longer allowed to do the thing you love. It's not a good deal.