Designing amps in conjunction with speakers

This is a question primarily for those in the community that commercially design amps, and perhaps those that commercially design speakers...

In your opinion, why do so few amplifier designers not also design and develop speakers (or at least work in close conjunction with speaker designers)?

If I was designing an amplifier, I’m want to specify the ideal characteristics of the “input” and “output” that allow my product perform optimally. The “input” option could be addressed using a balanced differential input (that follows the IEC spec). However, the “output” is a wildcard. I’d assume an amplifier designers has a target speaker (load) in mind. Why not also define that target (load) or work closely with a speaker designer to showcase the best the speaker / amp pairing?

We can take the obvious answer of “limiting the addrsssibale market” off the table already.