Because it takes a lot of time to modify. I did retorfits on my 2001 Dodge Neon, and to do it right and look good takes time. Especally if you are asking someone else to do it for you. If you are handy with tools and have the time, i would say do it yourself.
1. I had to buy a seperate set of lights so I can continue to work and drive my car.
2. Baked the lights to loose the adhesive to seperate teh lense from the housing.
3. I went the easier route and picked up the Missimottio (spelling is wrong) projectors that mount where the halogen bulb fits. I looks good and does its purpose being Bi-Xenon style. Others would modify with E36 BMW lights ect... More cutting and adjusting and time.
4. Adjust with lens off so the cut-off lines are level.
5. Seal the lens to the housing and reinstall.
It took me part time working on it over 2 weeks to disassemble / adjust / reassemble and then troubleshoot when the HID wouldn't start up.
If you go with a custom paint and lighting you can add upto 300-400 bucks extra for a better looking projector.
This has been my experience with retorfits. On a Dodge Neon the retrofits would run between 600 and 1200 (depending on the projector being used)