Why bath sonicators don't come with adjustable power setting?
All the bath sonicators I saw online don't have a power level setting.


This depends on the product lines. 

I've mostly used BANDELIN SONOREX SUPER ultrasonic baths, and indeed, the most simple models of this line just have a (mechanical) timer.

However, models of the SONOREX DIGIPLUS line do come with power control.

In the past, the ultrasonic baths were mostly used for cleaning purposes, while the probes, which typically had a much higher power, were used for sample homogenisation and similar purposes. Here, power regulation was apparently more important to avoid degradation of the same due to local overheating.

As usual, exceptions prove the rule:


I've used ultrasonic baths to support oxidations with $\ce{MnO2}$ or PCC on silica
Cleaning of modern (composite) materials seemingly requires power control, e.g. to avoid ablation