it was actually quite difficult to simulate a valve sound with a transistor amplifier in the days of analogue electronics. Could be done, but not cheaply, and needed a lot of circuitry.

It was not just the balance of adding lots of 2nd and 3rd harmonic distortion, which had to vary with signal level, but of simulating the clipping behaviour which was quite different from transistor amplifiers, and the high frequency losses and low frequency softness due to output transformers, which would all be signal dependant.

If anything, it was the smooth overdrive performance which was the main factor in the valve sound, since so many amplifiers were relatively underpowered and continually driven into clipping in use.

Nowadays, if you know what you are doing, it is easy, since formulating a dsp to simulate valve sound is no different to sampling an acoustic instrument or hall reverb, you just sample the valve sound!