Why are there no botique car audio recievers?

Me either, but I had a Bose system with what looked like a Pioneer  head
in a Nissan Maxima .
Sounded great !

One of the big problems right now that is depressing that market is the integration of the audio electronics with the rest of the computing power and electronics of the car.  Most of those systems are proprietary so the days of just swapping in a better head unit and set of speakers are over.  The factory amplification may employ built-in DSP & equalization so that you have to compensate for that, as well.  My BMW required an expensive "adapter" (nearly $1K) to go between the factory head unit and the aftermarket amplifier, which also had to have adjustable EQ built in to compensate for the difference between the response of the factory amp/speakers comob and the amp/speakers I had installed.  My wife's Audi does not even have a head unit per se.  The display is built in to the infotainment system,w the display functioning for GPS, audio, bluetooth/phone, etc with all of the controls on the steering wheel and on the center console..  You'd have to cut a hole in the dash somewhere to add a head unit and you know that ain't happening!  FWIW, her factory unit sounds pretty darn good. 

@swampwalker - nailed it. I just got my dad’s 2008 Infiniti EX35. Wonderful, fast, quiet, and fun to drive. But the Bose stereo system "Blose", even when used with a portable FiiO music player into the aux input. The problem is the integration. You cannot replace the head unit.

I think the best way to go, if you are spending some cash, is to find a good installer, one who focuses on sound quality, not dB levels, and use one of the aftermarket DSP units that flattens out the audio out of the head unit, and allows you to install your own amplifiers and speakers. You still have some reduction in sound quality from the head unit, but it’s the best you can do if you can’t remove the head unit.

Before the Infiniti, I had a Subaru Outback VDC with the McIntosh-designed, Clarion-built in-dash CD changer. No aux in, but it sounded excellent. Too bad the car suffered from high amounts of wind noise, so that at highway speeds, you couldn’t really appreciate the McIntosh system.

bondmanp, i share your pain--the bose system jacks up my otherwise excellent maxima. i've also looked into the aftermarket mods you refer to--they're very costly, tho i'm certainly tempted. the fender system in my wife's current vw, otoh, is really, really impressive.