Racecars are only really slowed down by two things.. budget and rules. Every year the cars get faster as teams budgets optimize the rule sets more and more. Often rules are created to slow the cars down, and to limit spending. If you let people spend unlimited money and have totally open rules, very quickly you'd have cars so fast they are completely unsafe on the tracks they are designated to race on.

MotogGP for example has tightened rules basically every few years to lower the gap between the top tier teams and the low budget teams, and to slow the bikes down. Even with that, the bikes top speeds are still rising and talk of unsafe speeds at certain corners grows every year.

If you just allowed 20% larger engine sizes and custom traction control, and variable aero, bike speeds would rise so high people would die every few races instead of every few years.