AWS/GCP are pretty much as good as anyone reliablity-wise, as long as your product is multi-AZ etc.
But SLAs are a joke - your business should be making $100 for every $1 you spend on compute, the fact they'll refund your $1 is trivial compared to your $100 loss.
I mean, they're an infrastructure company, not an insurance company. If your power goes out, you don't get compensated for the $1M deal you would have closed if only your lights were on. If it's that important, you have a backup.
I think the reality is that if stores are closed or websites are down, people will just come back later. Maybe not so much for ad serving (if you're bored at work and Reddit is down, you're not going to make time after work to go view ads there), but for actual businesses, it might not be as big a problem as people would like to imagine. But if it is, that's why you can spend millions on a multi-cloud solution.
It's kinda silly to suggest nothing important runs on aws/gcp, there's plenty of huge businesses running on them, with equally large legal departments ready to string amazon/google up by the balls if they breach the terms of their contracts around SLAs etc.