Basically it comes down to build variety and versatility. Those things take the "joy" of everyone in a racing game using the exact same car with the exact same build, either with no cosmetic customization because why bother or all the cosmetic customization because why not, and bring it into real life. Ethanol makes it far too easy to get stupid amounts of power from certain combinations (forced induction Ford 5.0, hybrid turbo GTI, and M340/440i come to mind), and drag tires make it too easy to launch that power. These cars become absolutely unbeatable in short-distance, straight-line races, at the price of being less capable, less fun to drive, and not even making their full power during the 90% of the time when you are not specifically drag racing. Meanwhile, the level of competition within drag racing starts to run away from your typical general-purpose/daily-driver build. Now say there was some sort of gentleman's agreement in place to stick with pump gas and cornering tires (and maybe also manual transmissions where possible), forcing people to make do with whatever power they can make on the former and handle on the latter. Suddenly a lot more cars can work, and going fast requires actual building & tuning, not just installing an off-the-shelf kit or canned tune on one of three meta cars. Even drag racing now requires some skill, and the cars in question become a lot more fun to drive the rest of the time. The lower level of competition might also mean some "wiggle room" for deference to aesthetics, so there could once again be a market for, say, body kits or a variety of wheel styles. Under such an agreement, I would say that if you can get a sub-5 0-60MPH, sub-13 quarter mile, 160MPH or better top speed, and 0.9g or better cornering all on the same parts and settings, you're doing very well for a custom build (note my own car is not there yet). Theoretically, if built and tuned the right way, most cars made since about... I dunno, 1955 maybe, except for kei cars and things with part-time 4WD, could probably hit this set of targets without resorting to LS/K swaps, ethanol, or other boring meta lameness. tl;dr Without drag radials, ethanol fuels, and modern non-slushbox automatics, we'd probably have far fewer copy & paste builds, less "iF yOu DoNt HaVe 1000hP yOuR sLoW" nonsense online, and in places like the US where the regulatory climate still leaves room for car culture to sneak through, maybe even better aftermarket support for a wider variety of makes & models. (If none of this makes sense, remember that traffic policing in my part of the US is basically the opposite of Australia, and we don't have vehicle inspections where I live either)
