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The majority of US made cars have an automatic transmission as standard equipment. Manual transmissions are available but usually by special order.
Car rental companies in the US tend to buy cars with automatics, since that is the preference of US drivers. And while yes there are European tourists who rent cars in the US and prefer manual transmissions, they are really just a small percentage of the rental customers. The reverse was true for many years in Europe in that only manual transmissions were available and Americans traveling in Europe had no choice but to drive a stick.
There are some smaller rental companies that may offer manual transmissions at some locations or on some models, but at airport rental locations, it is usually the biggest companies and they pretty much offer automatics.