Not an electronics expert here, sorry if it doesn't make sense but why is there a limit on symbol rate anyway? I understand tx power limits to avoid interference, but why an additional symbol rate limit too?
Because it is a shared resource. It is limited not to exceed the bandwidth of a “typical” SSB transmission.
Typical is in quotes because some people think it is cool to allow their radios to transmit up to 10KHz bandwidth because “high fidelity SSB” is apparently a thing.
The symbol rate is ultimately linked to the amount of bandwidth a signal uses, i.e. the amount of the radio spectrum that's used by a signal, rather than the physical area over which that spectrum is used (which relates to power).
