Why do most microcontrollers not embed a 32k oscillator?
To get &lt; 50 ppm accuracy the Q of the resonator must be &gt; 10k. This is done either with MEMs Ceramic crystals acting like a tuning fork or a Quartz Crystal ( too big)
This technology is not compatible with silicon chip materials or adds cost to a custom fabrication where it might not be needed.
However, if the 32kHz is just for low power and not high accuracy then the internal lower Q variable clocks are useful.