My understanding is that CPU overhead is the limiting factor but there may be other reasons too. Every chance the future will bring more layers. If they could today, they probably would

My fav use for layers is for wet reverbs with different timings and panning tricks, leaving layer 1 nice and present but 3 layers still fires my creativity enough esp. for pads and FX heavy presets with tweaks beyond piano realism.

Having even 3 layers means I go into my DAW way less since for those extra audio tricks.

Unfortunately, Pianoteq 7.1.0 only allows three instantiated layers, which is probably a reasonable maximum for conventional per-note layering, but it doesn't account for the above (ab)use case.

Since each instance can already be assigned to a MIDI channel, a higher total maximum (48?) layer count would make the Pianoteq standalone more useful.