It's a multitude of factors but basically they can act like that because they are dominant on the market.
The classic "nobody ever gets fired for buying IBM".
If you pick something else, and there's issue, people will complain about your choice being wrong, should have gone with the biggest player.
Even if you provide metrics showing your solution's downtime being 1% of the big player.
Something like Cloudflare is so big and ubiquitous, that, when there's a downtime, even your grandma is aware of it because they talk about it in the news.
So nobody will put the blame on the person choosing Cloudflare.
Even if people decides to go back (I had a few customers asking us to migrate to other solutions or to build some kind of failover after the last Cloudflare incidents), it costs so much to find the solutions that can replace it with the same service level and to do the migration, that, in the end, they prefer to eat the cost of the downtimes.
Meanwhile, if you're a regular player in a very competitive market, yes, every downtime will result in lost income, customers leaving... which can hurt quite a lot when you don't have hundreds of thousands of customers.