Why do I keep getting fuel in my crankcase? The filter is clean.

You carb float and needle valve are not closing completely when the engine is off. Drip, drip, drip the carb allows excess fuel to enter the combustion chamber when there is no combustion. Fuel seeps passed the rings and bingo, fuel in your oil. I'd start by cleaning or replacing the needle and the seat, make sure the float is not fuel logged.

Why do I get fuel in the crank case of my 12.5 horse briggs and stratton engine? The air filter is new.

Needle is wore in carb , or your have debri in carb hilding needle open.
Take carb apart clean and install new needle.
Luck