Here's an analogy: in a democracy, first-past-the-post voting is, in a vacuum, about the worst voting system that exists. But the reason that it was adopted in so many places is because it has one advantage: its sheer, utter, bone-headed simplicity. In a context where most constituents are illiterate and unfamiliar with the notion of democratic government, it behooves you to pick the simplest solution that can possibly work, even if it leads to worse results than more complex systems.
Hereditary absolute monarchy is the same thing, but for selecting heads of state. Who's in charge? The guy with the biggest army. What powers does he have? All of them. Who succeeds him when he dies? His firstborn. It's dead simple to implement, which made it an attractive solution in times before any semblance of mass communication. But in practice it means the quality of your head of state is totally detached from their actual talent at serving as head of state: the first guy in line was just good at leading an army, and the rest of his descendants are just randos who won the birth lottery. It's not a good solution unless you're willing to make loads of sacrifices in order to have the simplest system possible.
(And yes, of course, the UK is not currently an absolute monarchy, but you appeared to be asking in a general sense.)