Differential amplifier vs Instrumentation amplifier

I'm trying to understand what is the advantage of using a difference amplifier as opposed to an instrumentation amplifier.  Difference amplifiers have the problem of loading the signal,  and mismatched loading will create common-mode voltage.  This won't happen with an instrumentation amp.  The only things I can think of is a diff amp can be faster and has differential output,  and also maybe less expensive?   I wouldn't think there's that much difference though.   Why don't INA's have differential outputs?

Thanks   in advance!