A key problem is that microwave frequency bands are very broad. The Ka band, for example, is from 26.5 GHz to 40 GHz. That's extremely broad, and one could use a vast number of individual frequencies in that band, simultaneously in the same geographic area without individual users interfering with each other. So your radar detector has to scan the full band. Unless it's very discerning, it can't tell whether the sensed signal is from a police radar, an adjacent car, a supermarket with an anti-theft shopping cart wheel locking system, or a Predator drone. Scanning an entire spectrum (or a broad segment) is time consuming, and analyzing sensed signal to identify them is even more time consuming. The latter is also expensive.