Because they scare off the experts who post the really useful (i.e. practical) answers.
If too many of them leave, the site becomes useless.
The people who answer (and upvote) questions like "What's your favourite programmer cartoon" do not think this through - they just see something funny and click the upvote button. People have argued "but this got upvoted so it must be good" since the site started, but it just isn't true.
There's a world of difference between "I want to learn Perl or Python, what are the pros and cons of each?" and "What's your favorite programming cartoon?"
I think moderation efforts would be much better spent directing the answers to the open-ended questions away from monoglotist flame wars and toward an honest discussion (there's that word again) of something.
I don't think anyone would complain about aggressively closing questions like the one you posited.
You start attracting the people more interested in discussion than actually Getting Things Done. Bikeshed discussions become the order of the day, and why not, because they are so fun and easy to answer -- everyone's got an opinion to share. And they generate huge amounts of reputation because they're easy to process. "Yup, GOTO sure is terrible, preach it brother, {upvote}"
If too many of them leave, the site becomes useless.
The people who answer (and upvote) questions like "What's your favourite programmer cartoon" do not think this through - they just see something funny and click the upvote button. People have argued "but this got upvoted so it must be good" since the site started, but it just isn't true.