Still didn't play a down of football throughout college. Going from high school football to best TE in the NFL is the mark of an elite athlete. Brock played high school football and couldn't even make a third team/benchwarmer spot. Different breeds of animal.
You act as if this is some magical jump. Gates was an undrafted free agent, who had to work his way up the ranks (albeit somewhat quickly).
Lesnar had no real passion to play football, as he admitted. He was simply looking for a way to make a living outside WWE, having left the company after WrestleMania XX. Lesnar had a tryout with the Bucs after college, but he would have been on the practice squad. With his being deep in debt, due to student loans, Lesnar didn't see that as a viable option.
Vince McMahon came calling, and with the stroke of a pen, Lesnar became "The Next Big Thing" and his pockets swelled up as a result.
MMA/UFC is more Lesnar's nitch. It allows him to use his amateur wrestling background and compete and (most importantly) pay his bills, without the near-merciless toll on his body that WWE took on him.