Because turn order would be constantly changing mid-turn as people moved up the culture track. Or you would lose track of the correct order if you tried to ignore the changes until a new age.
That's not much of a reason. Lots of games have ways to record the turn order until it resets the next time.
The basic issue is some people prefer to always play clockwise because it's simpler to keep track of whose turn it is, and some people prefer to "earn" every position in the turn order. Designers often choose clockwise to cater to the people who don't want too much complexity.