This is actually pretty consistent across companies: being the most senior person around when the last manager leaves. Sometimes it's a senior dev, sometimes a PM, heck I've seen it be somebody from the UX org. The path of least resistance is a quick promotion from within. As many of us have experienced this leaves us with a non-zero amount of managers who are both unqualified and uninterested in their role, but accept it for status (you're supposed to get promotions after X years right?) and pay reasons.
what people don't undertand (and that included me, when I did the change) is that being a manager is a totally different job than being a developer. you can be an extremely good developer, but be a lousy manager. there are different skills needed for those roles and not all of them are transferable. when you transition from a senior developer, you don't transition into a senior manager, you start from zero. it helps to be a good dev, but it's not a guarantee for success.
This is a common misconception and it is exactly how "bad people" progress into a role where they aren't a match. It is an entirely different role. Most engineering managers come from an engineering background and rely on the learned experiences of that job in order to be successful.