Bug fixing in games and programs, take this from a programmer is painfully agonising at best. It is slow, horrible, and often very unrewarding work, trying to figure out the causes of some bizarre behaviour that isn't always even repeatable...it's hell.
Unrewarding you say? Very much so. You track down some obscure problem, something it may have took weeks to identify the cause of, get to repeat, figure out the exact problem, and via some trial and error patch. You feel absolutely chuffed, all that work and finally you fixed it. You want to scream to the heavens the wonders of your success. Hell, you may even play some victory music and dance around the room. You release the bug-fix. Most peoples responses are nothing more than a cynical, condescending "It's about time". You want to cry.
When fixing bugs, you have to prioritise (something that in itself can take time), and after a certain point the "time vs pay-off" ratio just doesn't justify fixing the problems. Also, some are so obscure and ingrained in the system to essentially be unfixable. Why do you think so many bugs in modern games wind up labelled as features?
At the end of the day, no program is bugless. None. It's a law of programming, if you think you've fixed every single bug, a new one will be discovered. Why do you think the Bugzilla tracking program deliberately reports Zarro Boogs instead of Zero bugs?
Hell, I once saw a quantum bug which actually didn't take effect until it was fixed (a freak occurrence where one change actually managed to hide the problems caused by the other change).
Not only that, but some bugs just aren't worth fixing.
And for one person to be expected to fix bugs that could take a whole team weeks to isolate the cause of and track down? Perhaps my programming experience has made me softer on other programmers when it comes to such things, but that's just unreasonable. Hell, that one person alone managed to make DF as it is now is mind-blowing at minimum...