it is bcoz you are not looking, MHR Team 7T 70cc is rated at 24hp, Evo2 is rated at 21hp, 2Fast 80cc is rated at 26+hp etc etc.
dude some guys racing a metrakit prorace setup making 20hp have more than a full season of practice & (circuit)racing & most of them are still running the original piston & ring that came with the cilinder. it is your ignorance that is making you think they would not last 1 race.
a lot of guys here also run MHR Team & Evo cilinders on the street & make over 20hp & do so reliably & they have never used a carb bigger than 28mm.
ok then why stop at getting a ninja 250, get a big bad busa which is the USA's #1 squidbike, anybody can go fast in a straightline. if you do indeed get a busa as your first real bike i'll gladly have you insured for more than a million $ with me as a recipient if you die.
learn to walk before you run.