Most long range air defense search radars operate in the L,S and C bands (1-2,2-4, and 4-8 GHz). Air/Air and Target Track/Fire Control radars are generally in the X band (8-12GHz). These radars are what the term ‘stealth fighter’ was designed for. RF absorption coatings, fuselage shaping, etc. are more effective there because of smaller wavelengths and better effectiveness of the coatings.
When you get to radars with wavelengths that are larger than the aircraft itself, about the only thing you can control the reflections with is absorption - the coatings if there are any, and at that frequency, the effectiveness of the coatings is not very good.
Yes, that is the heart of the issue. Being able to detect something and being able to engage it are very different things, and require very different radar wave-lengths.
If I go duck hunting with my eyes closed, I will still be able to detect the ducks with my ears, but my chances of shooting one are about zero.
Large wavelength radar systems require large antennas and these are HARM magnets any time that they turn on.