Why does each servo/tx/rx need it's own frequency?

There are many 9 channel transmitters available that will operate on DSM (no interference) and allow you to control each individual servo any way you want.
People use them for crawlers and aircraft constantly.

I have 1 customer who is ordering a 12 channel transmitter/receiver!!!

I know you won't reveal WTF you are trying to do but it would probably help a lot of people understand and solicit more help that way.

You may try ultimaterc.com or RC-monster.com

Good luck!