It's not that people don't know it's a SPOF. The issue is that if you fail in a way that is common, nobody blames you. Cell phone 2FA is so ubiquitous that when it doesn't work clients wonder if they're the one fucking up. We had a massive internet outage in Canada recently and nobody blamed individual shops for not being able to take credit cards, they blamed the phone company.
If you roll your own thing, even if it's objectively better, you're painting a target on your back. Now when your new thing fails it's your fault.
There's also something to be said for how modern financial capitalism has squeezed redundancy out of everything. Supply chains are just-in-time, businesses rely on a million vendors for everything, if a single link in the chain fails it usually has a huge blast radius because of broad consolidation in these upstream suppliers. Basically there are a lot of small businesses that depend on like 5 large companies (think Sysco, telecoms, etc) and if those large companies fail every small company fails in the same way, so there's no incentive to derisk (because your competitors will all also be failing in the same way).