As in, a huge SaaS company offers their product for self-hosting to individual companies, but it's not practical to self-host because the code is highly specialized for supporting hundreds of companies instead of just one? And it's hard to have an architecture that works well for just one and for hundreds?
For example Sentry requires ClickHouse, Postgres, Kafka, and Redis presumably because they were the right tools for their needs and either they have the resources to operate them all or the money to buy the managed options from vendors.
Also, the main concern people have with hosting Sentry is the sheer number of containers required but most of them are just consumers for different Kafka queues which again is presumably this way because Sentry ops prefers it this way, whether it be for fine tuning the scaling of each one or whatever the reason.
What makes sense for a SaaS company rarely translates to sensible for self-hosting.
They have all sorts of caching, autoscaling, distributed systems and other stuff thats complete overkill for all except that largest installation. Plus all sorts of software features only needed by a few customers and extra layers to be multi-customer.
It's the difference between a hoop in your back yard and a NBA Stadium