Indeed. So with monolith usually we already have 3-4 (or more) somewhat reliable systems, and one non-reliable system which is your monolithic app. Why add other non reliable systems if you don't really need it?
Making a system to be reliable is really really hard and take many resources, which seldom companies pursuit.
You'll also likely use multiple databases (caching in e. g. Redis) and a job queue for longer tasks.
You'll also probably already have multiple instances talking to the databases, as well as multiple workers processing jobs.
Pretending that the monolith is a single thing is sneakily misleading. It's already a distributed system