As far as I know, there are many amps with cab emulated outputs.
But the thread topic naturally is about "amazing" speaker sims.
The problem is their realistic implementation to amps, especially tube amps. With tube amps, especially bass amps, you either need to dumb down several hundreds of watts to a dummy load or figure out a way to mute the power amp reliably.
If the signal isn't flowing through the power amp + speaker path you loose a certain interaction (which amp designers have for decades tried to emulate with varying results) and the simulated output won't sound the same as the real deal.
Furthermore, most of these dummy loads and cab sims have "fixed" responses. Even if the dummy load is advanced and reactive it won't load the power amp with unique characteristics like each speaker cab would. Speaker sims usually have the same fixed response too and usually a highly realistic emulation of speaker response is way too complex to realize with analog circuitry. Most cab sims simply combine a bandpass-filter on some arbitrarily chosen frequency to mimic "free air resonance" and then a couple of second order low-pass and high-pass filters to simulate the reduced bandwidth of a speaker. If you take a look at response of real speaker you immediately see it is hundred times more complex and introduces all kinds of comb filtering from cone breakup nodes towards the high frequency limit in speaker's bandwidth.
So, is an amp manufacturer going to spend the same amount of money to realistic cabsim than they spend into developing rest of the amp or will they rather market the advanced cab sim as a separate unit?