Why wouldn’t a faster engine speed give more power?
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An engine is designed to work within a certain RPM range. At the top or bottom of the range, it makes less power. Outside the range, it doesn’t work at all. The range can only be so wide, given the nature of the system.
On the high RPM end, it’s usually because the valves don’t slam shut quickly enough to trap in the explosion. This is known as valve float. If the valves are open even slightly, it means that the explosive pressure escapes the cylinder instead of being used to push the piston down. It gets worse with higher RPM.
You can try to raise the limit, but not indefinitely. 10,000 RPM is considered the realistic upper limit for an extensively modified 4 cylinder engine (in a street car), usually bigger engines rev considerably less. 8500 rpm is considered pretty good for a V8, for example. However, at the top of the RPM range, it will always run into this problem.
You can put a stronger spring in there to get the valve to shut faster (allowing you to rev higher), but this makes the car lose low RPM power, and can even raise the minimum idle RPM needed to keep the car from stalling out. Different cams are also used to change the timing of the valves (try to shut them a little earlier) and also how far they open up. Cams that favor high RPM make the idle really erratic as well.
They can make engines run up to around 20,000 RPM, but they are for the most extreme race cars or motorcycles only (formula 1). They have to idle at really high RPMs (3-5000+ RPM) just not to stall out. It would be very inconvenient to drive something like this on the street, and it would be loud as hell too.
This doesn’t even consider the reliability concerns associated with high RPM performance, of which there are many.
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