I think you're right, that's why I wonder if Tesla will either make trailers or figure out a way to reconfigure existing trailers with regen brakes. Considerable benefits to be had in electricity generation and also trailer control in slippery situations I would think.
It isn't worth the expense. Most semis will never pull the same trailer more than a handful of times, so it would require adding regenerative braking to all trailers in order to really be effective. The system would also be useless when those trailers are getting pulled by diesel trucks, which means extra dead weight that eats into your cargo carrying capacity. Finally, most trailers just sit around for extended periods of time when being loaded and unloaded, which isn't a good use of the regenerative braking equipment.
Regenerative braking will be at least as powerful as current diesel engine braking, so I'm just not seeing a cost benefit to making the trailers more complex.