They aren't, necessarily. But there is way more effort put
into improving transistor performance, at more foundries,
by more people in the integrated than the discrete market.
Discrete devices have to carry the baggage of packaging,
bondwires and lead capacitance and so on. An integrated
circuit's individual transistor does not; that burden falls on
the higher level assembly and some small portion of the I/O
devices, but not the core.
Integrated devices optimized for low voltage will have
many parameters better than a medium or high voltage
discrete; few people make low voltage discrete devices
that could take similar advantage of tradeoffs.
Gummel-Poon is "good enough" for older, crude geometry
devices. It has nothing for many second-order effects
that appear only in aggressive lithography, heterojunctions
like SiGe, or other post-'80s device construction features.
Even so, even then, we were on Eber-Moll higher "level="
models for devices sub-GHz, tens of volts, similar to what
a discrete device of the day often looked like.
In the end you're probably not the guy who's going to
pick or fit the model, and you get to use what the foundry
gives you.