Why do creationists believe the world was created 6k years ago?

Well there is a way to get this figure from the Bible, but you need to do some work.Basically, you start with Adam and, by using the genealogical information you arrive at a time span of 4004 years from Adam to Jesus, then add on the 2007 years since Jesus was born (yes I know they got the wrong date), then you essentially have a creation of the universe around 6000 years ago.The guy you want to look up is Bishop Ussher, he arrived at the creation date of 4004 BC.There's some info here http://www.lhup.edu/~dsimanek/ussher.htm

Is there any good reason to assume that before day 5 the days were not 24 hours long?

And they actually do the Bible itself a disservice.