Then they would be lost. Comments are for understanding how something works. TODOs are for understanding how something might not work.
Edit: And if you put them in a tracker, they'd be distracting and confusing for team members less intimately familiar with the codebase, e.g. a PO. You could also choose a word other than "TODO", as long as it won't produce a ton of false positives in a search.
If you put them in a tracker, soon enough there will be somebody asking why 90% of the issues are not being worked on, will complain if they just disappear one day, and will disappear for political reasons.
