Need to adjust the speed every time I turn on turntable

This is a common problem with BD tables. It is caused by a combination of belt slippage (which can be cured with proper tension) and belt creep which cannot be cured, but can be compensated for with feedback. No matter how accurate the motor speed, without feedback the platter speed will vary over time due to a number of variables that cannot be controlled.

SOTA now sells a PSU and tach with feedback that will keep the platter on speed regardless of variables such as belt diameter and durometer, stylus drag, bearing oil viscosity, temperature, belt creep etc.
https://sotaturntables.com/product-category/eclipse-series/

AC motors are locked to the drive frequency, but the wall power frequency does vary a small amount over short periods.  This will manifest itself as a wavering on single notes, but will not show up as speed drift over longer periods of time.  Long term speed drift is caused by changes in the belt and bearing lubrication over time and changes in stylus drag from the beginning of a record to the end.

The speed of DC motors is more difficult to control than AC synch motors.  Even with tight DC regulation of the voltage, the motor speed will be affected by torque load and temp.  Some of the OL motor controllers compensate for speed changes with load variation (stylus drag), but without temp compensation, the speed of the motor will still wander.
A synthesized PSU will lock the speed of an AC synch motor to a quartz reference, but as I posted above, this does not guarantee constant PLATTER speed.  Even with perfect motor speed control, the platter speed will drift over time.