Quite scary what kinds of causes trigger such incidents. As a frequent flier, I would have never imagined something like this.
How are there no regulations that should have prevented or caught this? I bet there will be now.
Commercial aircraft are required to be designed to maintain 15,000 foot equivalent cabin altitude after any probable failure, which includes multiple missing windows, per the requirements of 14 CFR 25.841 and it's EASA equivalent.
There is already regulation in place that should have caught this but did not. The captain is required to perform an external visual inspection of the aircraft before departure, which follows a checklist that includes looking at the windows. It appears that did not happen.
We've no idea if the damage was visible prior to takeoff. It may have easily passed a visual inspection on the ground before moving at speed with a pressure differential. As per the article at least one of them appears to have detached in flight and struck the horizontal stabilizer.
Also the engineer who signed off the aircraft will do a walk around.
The location of these windows might have made it hard to spot the damage.
I would think a pressure check would be trivial. I would also assume a visual inspection is part of takeoff (camera or human). I assume this sort of thing is rare enough to not warrant the extra time/cost of these trivialities.