Or they could just limit the number of refunds you can get, to say 1 out of 10 games that you buy, and no more than one per month.
Or they could just refund you any time you want. Yeah, sure, a few people will scam them that way, but it's not like they have a huge per-unit cost for delivering the games to you. If it'll make users like the above more happy and more willing to buy from Steam, it's probably worth it in the long run.
I think your underestimating the number of gamers who would pull all-nighters to play through a game as fast as possible in order to play games for free. Heck, plenty of people already do that without the ability to get a refund once they are finished.
Hell, if I was still in college I would seriously consider doing it as a broke student.
Allowing for refunds within 24 hours is effectively a free 24 game rental for any game in the library.
I guess the fine line from Valve is, there isn't really any way to differentiate people legitimately having problems vs. people trying to scam the system, so their default policy is if you file a chargeback your account is going to be suspended.