They kinda do, the refit Galaxy can go to warp 13!
Good to know! I'm guessing you're of the opinion that the transwarp the Excelsior's captain talked of is just what we consider the TNG warp scale? Or is it a case of an actual transwarp (as we understand it today) experiment that would always fail but the data generated from it being enough to cause a massive refinement to warp field dynamics understanding.
Per Encounter at Farpoint it could go a bit faster, perhaps 9.6 going by how there's no attempt to go faster after Q's ball accelerates to 9.7+*, though at 10.2 it'll still be 544c faster than the canon 9.4.
*and, iirc the tech manual gives 9.6 as being sustainable for 12 hours.
I was actually curious enough to go looking for the reason why the scale was changed and they did come up with a decent in-universe explaination:
"By 2300 many were becoming increasingly dissatisfied with the Cochrane Scale. Although convenient for those using the warp formula, it was of limited use to Engineers and specialists since it took relatively little account of the interstellar conditions at the moment. Thus it took a great deal more engine power to achieve a speed of Warp 5 while within a gravimetric distortion than it did while in relatively "calm" interstellar space. Engineering departments lobbied for years to bring in a new scale, but the bridge crews resisted and Starfleet Headquarters - primarily composed of ex-bridge officers - concurred.
The loss of the USS Wilmington with all hands during an Ion storm in 2309 changed this attitude. It emerged during the inquiry that Captain Lamarr had seriously over stressed the Wilmingtons engines by ordering Warp 7 while within the storm; although the Wilmington was quite capable of maintaining this speed under normal conditions, during an Ion storm it was far too great a load. Although other factors contributed greatly, such as a serious breakdown in communications on the ship, Starfleet was unwilling to chance such a situation again."
"The Terrance-Neltorr Graduated Scale was first suggested in 2298 by two civilian warp field specialists of those names. On the TNG Scale the warp factor is indicative of the subspace stress levels which the vessel must both create and endure, rather than the actual velocity of the vessel itself. The actual speed denoted by any given warp factor would depend upon the precise conditions prevalent at the time. So a Captain using the TNG scale would be able to order Warp 7 while in space, a solar system, or an Ion storm and be assured that he would not be over stressing his engines. The new scale was also tweaked to accommodate a number of technical advances made over the last century and in development at the time. Starfleet conducted a quick assessment of several possible new warp scales between 2310 and 2311 before formally adopting the TNG scale, with the changeover made in 2312."
"For ideal conditions, such as are found in interstellar space, the speeds of TNG warp factors are calculated using either of two formulae:
Up to Warp 9: V/c = WF
(10/3)
Which is very similar to the Cochrane Scale. Beyond Warp 9 the formula becomes somewhat more complex. It is best approximated by :
V/c = WF
[<(10/3)+a*(-Ln(10-WF))^n>+f1*((WF-9)^5)+f2*((WF-9)^11)]
Where a is the subspace field density, n is the electromagnetic flux, and f1 and f2 are the Cochrane refraction and reflection indexes respectively. Under ideal conditions values of a = 0.00264320, n = 2.87926700, f1 = 0.06274120 and f2 = 0.32574600 can be expected within a "normal" area of deep interstellar space."
Ok, even for Trekkie nerds, this is an extreme level of in-universe backstory .
