Thank you for the answer to my question on strengthening the crownlands but it’s not really what I was asking. To clarify I’m wondering why the King didn’t strengthen his military might by taking lords sworn to the different Lords Paramount and have them swear fealty directly to him. For example, Harrenhal has been granted to numerous people by numerous kings. Why did he not keep their vassalage instead of transferring it to the Riverlands?

Ah, I see. 

I think what you’re running up against here is that feudal politics don’t work like nation-state politics. 

Sure, the King could expand the Crownlands vis-a-vis the other kingdoms (he already did with Massey’s Hook and the southern Crownlands across the Blackwater), but…unless he’s going to rule them himself (and that’s not easy to do – you need bureaucrats to manage your manors, you need bureaucrats to keep records, you need bureaucrats to pay the taxes, you need soldiers to make sure people don’t rob your tax collectors, etc.), he still has to give that land to someone in exchange for their fealty. Sure, you could get rid of one layer of subinfeudation, but that’s a huge political effort for not really that big of a change.

Moreover, and this is the real kicker, a king is supposed to be open-handed, a ring-giver. Indeed, giving stuff away is the primary way you get armed men to fight for you in a context where you don’t have the state capacity for a standing army. So a king who gets a reputation as miserly or greedy is going to find themselves lacking in armed men to fight for them no matter how much land they control. 

That’s the catch-22 of feudal politics: you have to give away the thing that people want from you to get them to do stuff for you, but the more you give away, the harder it is to get them to keep doing stuff. And historically, while kings did eventually grab more and more land for themselves (hence the coalescing of nation-states from nuclei like the

Île-de-France), the main route that kings used to increase their power was to convert feudal military service into taxes paid in cash (the so-called scutage) that would allow them to hire mercenaries and other professional soldiers, gradually building up the state capacity so that they no longer had to rely on the old way.