Because you can naively iterate through a million items faster than an additional network round trip would take.

So a lot of code quality debates don’t matter for the typical enterprise app. While a dev spends their afternoon shaving off 100 nanoseconds in the hot path, a second developer on a deadline added a poorly thought out round trip that adds 800milliseconds.

This architectural problems are also more difficult to unwind later since they tend to have cascading effects.