The AMA does have rules for advancement from C to B and from B to A. They post the list of these riders that have been advanced by their racing results on the AMA website. The problem lies more with other sanctioning bodies that do not have a clear advancement policy and never force riders to move up and these sanctioning bodies actually control the rider's racing in many parts of the country - Not the AMA.
So while the AMA is doing their part to move riders up and having rules in place, other people are not.
And Lap Times are not a true indicator of what skill level you should be running in. If you wanted timed racing, you should take up drag racing in the bracketed divisions. You get disqualified if you go faster than your division is allowed to go in the bracket.
Think of it this way, you are born with a certain amount of natural talent on a scale for your speed. With lots of work, practice, training, experience, bike set up, etc, you can increase your natural talent speed by a small amount. In the same way, injuries, poor shape, overweight, lack of experience, and intoxication can slow down your natural talent speed by a small amount too. But this represents the range of speed that you as a rider can have and looks like this on a scale:
SPEED RANGE
<-------------------------X-------Y------Z------------------------------------>
Another Rider
<---------------------------------X------Y------Z----------------------------->
Pro Rider
<-----------------------------------------------------------------X----Y----Z->
The X is your unexperienced / out of shape speed.
The Y is your average natural speed when you have enough experience to know how to ride and have everything working properly.
The Z is your peak speed attainable if you dedicate your life to motocross and train for it, eat correctly, practice all the time, and have plenty of money to spend on the bike that works best for you.
A pro rider has a much higher natural talent set / speed from the start, so if he starts riding at age 15 in the C class, he will be faster than guys that are training and everything with a lower natural speed who have been racing for years. Think Jeremy McGrath who started late after a long BMX career as a kid, he had incredible natural talent and won the C class when he started out because he was fast! Not because he sandbagged in California for 7 years before deciding to go to Loretta Lynns.
You do the best you can and race where you fit in based on the classes offered. Some riders will never advance past C level, some riders will never advance past B level, and some riders are lucky enough to have PRO speed naturally and will make a living doing what we all love to do for fun, just like any other sport. I can tell you that if I played football everyday for my whole life and trained non stop, I'd never be a NFL Lineman because I don't have the natural size / strength to compete with those that are.
Just have fun and enjoy racing against people that are your same speed no matter what they "call" it.