That accretable material is a natural result of rubbing on metal to metals etc. It then exists in an extremely high velocity field at tip speeds. The result is that it becomes a birthing field for flow eddys and blade tip stall.
You don't want accretion and you don't want blade tip wear, so you go for either open clearances of the early jet engine years or an abradeable case that lends itself to overhaul repair.