Because the weather is very changeable. You may get a lull in the wind for a couple of mins, enough to land.

I've been on a couple of flights like that. Once where we did two attempts and landed on the 2nd, the other where we did 3 but the had to divert. Other planes were just managing to land in the winds before and after our attempts.

The other problem is (as I found out on that flight) that mass diversions are not good. The airport I diverted to in the UK had dozens of unexpected arrivals, late at night. There wasn't the ground staff to manage this so it took forever to get people off. It then was too full to accept any more landings, so further flights had to get diverted further and further away.

So, if you did a blanket must divert you'd end up with all the diversion airports full (even to flights that could have landed at their original airport) and a much more dangerous situation as your diversions are now in different countries.