Why are there so few double DIN kits available and why so much money???

I am debating on going single DIN since they are cheaper and more readily available, this way I can always get a stereo with a retractable screen and still have touch screen dvd etc.

Does anyone see a downside to a double DIN unit vs retractable screen?

Also, is there a wiring harness available which will make an aftermarket stereo plug and play with stock speakers and AMP in a non-DSP and non NAV X5 such as mine?

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2003 X5 3.0i
Premium & Sport package
Steel Gray Metallic
55K miles and counting