Why do some people spell the word kluge as kludge? I see this mistake by a lot of people who should really know better.

It is kluge (pronounced "klooj") not kludge. Kludge would rhyme with fudge, judge, budge, etc. There is no other English "udge" word that's pronounced as an "ooj" word. There are lots of "uge" words, like huge, refuge, centrifuge, and deluge. See a jargon dictonary for more details.

Among Google-indexed glossaries, 4 say kluge and 9 say kludge. 1 lists both equally. The misspelling is common. (BTW, this Google glossary feature is quite useful, even if it does reproduce popular mistakes.)