Because the apparent miscasting of Arnold Schwarzenegger is actually very purposeful.
At the beginning of Total Recall, Douglas Quaid is presented as an everyman. He's got a wife, a tiny apartment, and a blue-collar job working construction. He's basically a futuristic version of Walter Mitty, right down to the dreams of high adventure and romance. So it might seem curious that Quaid is played by Arnold Schwarzenegger, at the absolute peak of his career as an '80s action superman. But part of the fun of the movie is that Schwarzenegger spins the whole movie sideways into near-camp: Semi-existential lines like ''If I'm not me, who the hell am I?'' become, in Schwarzeneggerese, ''Eef I'm nawt me, who da hell am I?'' More importantly, Schwarzenegger is a vision of muscular invulnerability but that invulnerability is constantly undercut in a movie where the villains are often attacking his mind, not his body.