Theory: Challenges are temporary because they are tools to collect gameplay data

They are already collecting data from how people are playing, simgurus have shared in the past that they are collecting how many sims people play with in a household and things like that, so I don't think there is anything specific that they are collecting with challenges.

Like some people have mentioned, I think the main purpose with challenges is to increase player activity, this is why they are timed. Challenges helps them with having daily, weekly and monthly players.