Playing as other characters

this is my first time playing borderlands…i saw a video of creators saying people like to stick with the character they choose, they rarely switch to new characters…i would disagree…however they way you have designed this game i can see why no one wants to switch characters…why the hell do i want to play this campaign 8 times to unlock every character at tvhm mayhem 3…ive now unlocked a second character to mayhem 3 normal but i sure as hell am not playing it again at THVM mode…why the hell dont you let us play other characters at tvhm mayhem 3 as long as we have unlocked on of our charaters…i want to play as every character but i have a full time job with overtime and cant play that much.