Starlink is cheap because it is tons of small transceiver satellites. A radar is more than a radio, it also includes time of flight ranging data (which requires high speed high accuracy electronics with accurate clocks), frequency data (to detect Doppler shifts) and transmits a complex waveform rather than a simple FM signal (which requires a sophisticated waveform generator). Most of all, the transmission has to travel through 100's of km, and then the extremely weak signal detected again. In the case of SAR satellites, there might need to be on-board computer processing to store multiple radar returns.
All this complexity adds weight and size. When you launch, you are literally paying the launch costs as if you were burning an equal mass of gold, plus Starlink only lasts 5 years. You can't just throw away a 200 km range radar every 5 years, especially in Starlink numbers. We're talking something like throwing away every single radar in the PLAAF then buying them again every few years. But if you go higher in orbit to lengthen the lifespan, the radar capabilities and costs both increase. Definitely not disposable.
Combined with optical though, and you have persistence + range + high revisit time. Optical is still the workhorse because its easy to interpret, can be put in high orbits like SSO or even GEO, and can find things that you actually can't find with SAR like wakes.
