Shouldered mortise-and-tenon joinery is the most time-honored method of quality cabinetmaking for some very good reasons:

Even without glue it forms an extremely strong mechanical joint with superior strength to resist shear, compression, and racking forces. Then add durable glue and a snug fit and its side-grain to side-grain shear resistance is multiplied.

In addition, it cannot twist out of place with "rail spin," unlike other frames built with crude butt joints and single screws.

The resulting hardwood frame provides a rigid skeleton, providing extra support for the rest of the cabinet, and even the countertop.

Case parts (including the corner blocks) are glued and toe-nailed into grooves for the same long-lived mechanical advantage.