You are vastly oversimplifying the problem here. One of the most obvious being, cameras don't really work without light on the subject being captured, and currently Starlink doesn't have a sun synchronous orbit like most (non-SAR) remote sensing birds do. Starlink would need to be completely reengineered from scratch to add even mediocre remote sensing capabilities without degrading the original mission of the Starlink platform (something akin to space mesh networking).
Put simply, it's MUCH easier and cheaper to build one type of satellite that does one task very well than it is to do two tasks poorly.
SAR typically are in sun-synchronous orbits too, but mostly for power management reasons. difference is, of course, that they do not need to have their ascending node in daylight.
the only SAR constellations I know aren't in SSO are Israeli (due to launch geometry constraints) and NRO's birds (LACROS/ONYX/TOPAZ, whatever their names are) [1]. some of them have exotic retrograde orbits [2].
Why is it a problem that it's not in sun-synchronous orbit? That just means that only a fraction will be usable at any given time? It seems like they have massive redundancy to make up for the lack of specialization?
Put simply, it's MUCH easier and cheaper to build one type of satellite that does one task very well than it is to do two tasks poorly.