As someone who drove trucks and was licensed in California - California has other rules that limit, including axle to tandem limits, and not allowing triples or turnpike doubles (both of which are allowed elsewhere, but not federally required). The unlimited length of tractor has practical considerations too, there are reasons why 95% of the long haul fleet is conventional condo sleepers (notwithstanding the folks who drive a RV style sleeper, with a little living area in it) - and 95% of the local fleet are conventional day cabs, mostly driver preference.

So while yes, I used California's state limit as an easy from of comparison, because it is one of more restrictive western states - you're correct in that the STAA allows a longer tractor. In the end, the rules are lightly enforced - but the state limits guide the total length of combination. It's why (for example) you almost never see doubles being pulled by anything other than a day cab.

Bit of a shitshow of authorities for any given piece of road but generally speaking it's the city streets and town-level jurisdictions that get restrictive moreso than interstates and major highways.

There is also the weight factor - even if you could legally have 90ft trailers and 25ft cabs, it would probably put you over the weight limit for many roadways and negate the savings.