"640K ought to be enough for anybody." - Bill Gates, 1981
The difference is human reaction times and the average driver. That, CAFE regs and the rise in price of oil.
1. The average drive just wont be able to use 600hp physically. Computers have reason to continue to follow this uptick, there are actual gains for the end consumer when increasing tech specs. Physics don't work that way. The sole purpose for SAE certified HP in a family car is acceleration and top speed. Once we get to a point, say under 10 seconds to 100mph, there will be little demand for increased acceleration and with speed limits around the world largely conforming to ~60-80mph, there is little need for higher top speed.
We could go to an unlimited speed limit system, but people have enough issues as it is in current society (though arguably excluding some societies like Germany) where people just are not safe at extreme speeds.
2. If we ever transition to a safe, autonomous driving system, even then something like 1000hp would be a waste because it starts to impact humans physiologically. To put it bluntly, it's just not comfortable to most people to go from a stop to highway speeds in 2 seconds as part of a daily trip over and over again year in, year out.
3. There's only so much you can squeeze out of thermodynamics. Even if we transition from a crude oil economy to something else, the usage rate would just be too high and expensive to maintain worldwide. If we hit say 90% thermodynamic efficiency with any fuel (the current average is somewhere around the low to high 30%). The only way that would be sustainable is with a truly renewable resource and you run into all the issues that you usually would with that even with a solar->water->hydrogen system cost, logistics or practicality wise for a 1000hp car that will be under WOT or agressive driving conditions.
4. We could do say 1000hp in a car that is driven conservatively, but then there's no point. It costs more to build a drive-train that is capable of that if you are not going to use it.
Will sports cars and enthusiast vehicles reach 600+hp commonly someday? Probably, but it won't be a commonplace thing in family vehicles.