I have an uber awesome amp, why buy a speaker with a built in woofer sub-woofer amp

Most people have enough application for their main speakers, and usually don't have an extra amplifier to drive a sub.   So that's one reason.   The other reason is, by integrating an amplifier, The manufacturer can control  The final product better.  The integrated amplifier in the subwoofer may not be winning any awards per se, but it should be well chisen to get the desired results with that particular sub.  There are manufacturers who make passive subwoofers, but they are definitely less popular. Wilson audio makes two subwoofers,  and I believe both are passive! But be prepared to pay a fortune for them.   In the end I think an amplified subwoofer is not a bad thing despite your concerns over the quality of the amps inside.

Being more convenient having all in one bass unit is an attractive selling feature too. You might have trouble finding passive subs since most of them are plug as complete all in one units. Maybe its time to do-it-yourself subs and buy a pre cut box kit. Some assembly required, or build one from scratch if you have the means to. Then its just a matter of finding a subwoofer driver you like. Dollar for dollar you can get ahead of the game this way and have more performance with some careful planning.

This place has well thought out structurally good and ridged CNC cut flat kits as I was mentioning they just need assembly, ported and sealed versions.

http://www.diysoundgroup.com/sealed-subwoofer-flatpacks.html
http://www.diysoundgroup.com/ported-subwoofers-1.html

A very basic reason it it allows you to use a lower powered amp to run the mid and high frequencies, typically a tube amp often SET.

The built-in sub amp only has to power the low-end frequencies, and is voiced for that specific speaker. It doesn't have to perform across the entire frequency spectrum as your power amp does.
 As a result, your amp now has less of a load to drive. Your amp won’t have to deal with low impedance demands.