Why remote controls are seldom used in Technic sets?
Many Technic sets are small and wouldn't fit motors or need remote controls.

Beyond that it is an extra expense which they need to leave out to make things available at various price points.  In some cases LEGO makes the motor optional to keep costs for the initial kit lower.  And the remote control adds complexity beyond that.

But the beauty of LEGO is that you're not stuck with their designs.  You can add these features yourself if you're motivated enough.