I don't think people, for the most part, are buying tix as an investment. I just think that the stadiums with fair weather fans, especially the ones with teams under-performing, tend to be that way. Teams like the Bills, Steelers, Packers, Jets, Eagles, Browns etc, with die-hard fan bases, rarely get taken over at home.
The casual fans of teams like the Panthers, Vegas Raiders, LA Rams, Tampa, Atlanta, Jacksonville etc, start to rationalize that they can make a little money, and save the money they would spend at the stadium, and watch the lose from the comfort of their own homes. They get to charge a bit more when the die-hard fan bases come to town, which makes it more tempting for them. Hence the big crowds for the away teams.