Not a game dev but I've done some programming and I love games so I'll take a stab. There's a few reasons I can think of:

1. That's how the engine they're using works. Game engines take a long time to develop, and so if you're using one off the shelf or from a previous project, it may be from a time when tying behavior to the frame rate was a low overhead tool for timing that would cause few if any issues. Given that Wolfenstein is a Bethesda title and they've made many games with similar engine level limitations, this seems most likely to me for this particular case.
2. They never intended to release it that way, and just set it up that way early in development to start getting to the real gameplay work. Then the deadline came around and it wasn't a high priority in terms of getting the game out the door.
3. Probably doesn't apply to Wolfenstein, but for indie games that have one or only a few developers, none of those people may have done much programming before, instead being more focused on other aspects of game design. So if you're learning as you go, there's a good chance some hacky things will make it in to the final product.