My guess would be: it's because memory management before MGLRU was really not good and required different userspace solutions and tinkering. You either get killed with OOM (no swap) or got into thrashing (swap).

And now, with PSI + MGLRU, situation is much better, but there are still missing features/subsystems which would be nice to have. For example there's no simple way to lock memory mlockall-style to ensure that rarely used daemon would not face long no-cache-latency upon accessing the first time after long idle time.