Why was primogeniture a thing? Seems a little inefficient. Why not simply give the lands to the most capable son?

It’s actually hugely efficient – birth order is a very simple and straightforward standard, it keeps the estate intact instead of constanty subdividing the land amongst the children, etc. It doesn’t always work out, sometimes the eldest son is a poor choice, but it at least is a system people can agree on – which cuts down on succession-related violence. 

Here’s the problem with giving the land to “the most capable son” – in most cases, you’re not going to get consensus about who that is, as the other candidates are hardly likely to agree that they deserve to be disinherited. So you’re going to get constant competition between the children, you’re going to have fathers and mothers and other kinfolk lobbying for their preferred candidate, and inevitably someone’s going to decide to make the decision a lot more easy by killing all but one son.