Availability is trickier than scalability. An async replica can lose a few recent writes during a failover, and a synchronous replica is safer but slower. A company using some platform might not even know which one they're using until it bites them.
99.9% of companies also aren't going to feel the performance difference of synchronous replication.
That being said, the setups I typically see don't even go that far. Most companies don't mitigate for the database going down in the first place. If the db goes down they just eat the downtime and fix it.
