Why don’t the people who produce the products just give the best possible results?

It takes time to stress test and find out the highest clockspeed possible for every single chip. It's impossible to do that for every single chip.

Instead they create play it safe and set the clockspeed that has the least failure rate or rather 99.99% probability of working for every single chip.

I mean intel does sell pre-binned chips that have been tested for high clockspeeds, but that costs extra

You can tweak and mod a cheap 1993 Supra to beat a ferrari. I mean nissan could do that for you, but it's not worth it for them to do so, spending thousands of dollars and hundreds of man-hours souping up a supra.