People managers rarely do engineering work as well - it's very hard to both manage people well and contribute significantly to the code for a project. It also sets up a sort of perverse incentive where you want to look good as a coder by taking on the glamorous, high-profile programming tasks, but this can actually demotivate your teammates because they're left with the drudge work. You don't want to be in competition with your reportees.

They're often drawn from the pool of former high-ranking engineers, or at least the good ones are. It's a very different skillset though; part of being a good manager is knowing when to step back and let your reports handle things, and the worst managers are those that haven't managed to separate their engineering ego from their team building ego.