Only because the database is on the far end of a network hop, and then all too often its storage is remote from its compute as well. This is the most common scenario in enterprise or cloud deployments, where for good measure a couple of firewalls, load balancers, routers, and reverse proxies are added at every stage.

In my experience, scaling is limited by database throughput, not latency.

Depends. Most apps that are slow have shitty app code with N+1 queries. If you can get past that stage and write good enough app code then eventually you’ll hit database throughout issues.

well, db latency in some naive unoptimized app with multiple sequential queries directly affects _app_'s throughput

But database performance is also a lot about CPU and RAM speed plus concurrent use of those resources.