Issue trackers are often politicized. You get projects where they auto-close stale issues. Or where they're resistant to creating issues that they know that realistically nobody is going to work on this in the foreseeable future.

TODOs are an excellent place for developers to describe where they'd like to go with a given pile of code if they had the time, and realistically that kind of work isn't suitable for every issue tracker depending on who has access to that issue tracker and who has opinions about what goes in there.