[tus88]

For the same reasons it has control surfaces at all. Planes need to adjust their aerodynamic properties in order to adapt to various external and intrinsic factors. Trim in particular maintains aerodynamic stability in the presence of some conflux of the two.

The trim equivalent of MCAS would be some elevator trim that ensures the excess AOA potential of the MAX could not occur, if set correctly. That is how a stable aircraft behaves. MCAS is a computer override to compensate for a condition that should not really occur in the first place.