And syncing databases is just waaay harder than having a dedicated one. It really seriously is a pain to configure and debug and manage storage and performance requirements around and dealing with scaling and managing failure modes and so forth. If you can even find someone who has the skills to do it properly so you never lose data.
It's far, far easier to just write normal SQL queries that don't have to make hundreds of requests, than to deal with syncing a database across all your webservers.
Why do you need 2 webservers (assuming that the cpu / network / disk throughput of a single server is sufficient for your application)?
And because designing for 2 is really different from designing for 1, but not much harder if you do it from the beginning. But expanding from 1 to 2 when you unexpectedly need to because of sudden traffic, and having to re-architect it, can be really hard.
I'm not talking about hobby sites here, I'm talking about anything designed for a business, where downtime of a day is unacceptable.
Databases are astonishingly more optimized and predictable and reliable than web servers.