Why is it doubtful? This is one of the largest and wealthiest cities/regions in North America.
A billion+ is on the upper end of things at the moment. Cowboy Stadium cost the equivalent of 1.4 billion if it were built today. It is absolutely extravagant and seats 80 000, which is 4th most in the NFL. That is not a comparison for Toronto. Heinz Field in Pittsburgh is considered amongst the nicest (despite the poor field) and it cost 270 million or about 400 million if it were built today. The league requires new stadiums to be 65 000. Soldier Field in Chicago has about 63 000 and has the most expensive tickets. That's more MLSE's style.
Anyone wanting an NFL team has to have deep pockets and be willing to pay for much of the cost of a stadium. A community like Vaughn or Milton could possibly offer some incentive to land a massive project like an NFL team. Those who own MLSE have very deep pockets.
Why are people marks? The NFL has grown incredibly and shows no signs of stopping. If you have the money to invest it seems like a sound investment. This isn't opinion or anything. In 1995 expansion teams paid $270 million. In 2002 Houston paid $700 million. The league has grown since then.