Rails is popular as startups because it's easy to pick up as a first language meaning the labor market was cheap and flooded with coding boot camp grads. Startups love cheap labor and they don't really care if things are built to last or will scale. They only care about it lasting until their next funding round or (hopefully) their exit.
Rails Devs are well compensated...where are you getting this from? Yes rails is more suitable for rapid development than spring, and easier to teach and learn. How is that bad though