In my experience it's not infrequent for bugs to unknowingly only get half-fixed, not realizing that the true problem actually lies a level deeper, or has a mirror case, or whatever. Maybe a good example is that a parameter to a command is 0, the bugfix sets it to be 1, but a later bugfix changes it back to 0, when the correct bugfix would set it to be 0 in some cases and 1 in others.

And that if you fix the bug without a test, then the second related bug crops up a couple months later, and somebody else tries to fix it similarly naively, and can wind up re-introducing the first bug if there isn't a test for it.

Basically, in practice bugs have this nasty habit of clustering and affecting each other -- if the code was trickier than usual to write in the first place, it's going to be trickier than usual to fix, and more likely than usual to continue to have problems.

So keeping tests for fixed bugs is kind of like applying extra armor where your tank gets hit -- statistically, it's going to pay off.