Probably because no one can keep up with the life and character of the fans. The mascot would constantly be outshined by them so their job would become obsolete. In a way, the fans are the mascot(s).
The Oakland Raiders don't have a mascot. The pirate is just the symbol, but it's not the mascot and it does not have a name.
Red Raiders And there mascot is The Raider
raiders
raiders
The Oakland Raiders did not have an official mascot in the traditional sense, as the team has historically focused on its brand identity centered around the "Raider" persona. However, the team has had various unofficial mascots and characters over the years, with the most notable being "Raider Rusher," a costumed character who represents the team's spirit during games. The Raiders are often recognized for their distinctive silver and black colors, the pirate-themed logo, and the tough, rebellious image associated with their fan base.
A mascot is someone inspirational that is on the sideline or at court side in a event. He/she is basically cheering for it's hoe team.
According to this contributor's quick look around the internet, no major team has a pitbull as a mascot. Several sports team use a bulldog mascot, though.
They have a mascot because every team wants and likes to show school spirit.
The Ducks hockey team mascot is Wild Wing the Duck. He has been the teams mascot since 1993. The name for the Ducks mascot was chosen from a 'Name the Mascot' contest.
Sports teams from Oakton Community College are called the 'Raiders'.
The blue jays mascot is Ace.
Mr. Met is the mascot for the Mets!