But it ignores the extent to which we've deliberately worked very hard to make capital mobile. As noted elsewhere in the thread, feudal societies used to see most wealth accumulated in the form of real estate, which is more or less the definition of immobile capital. We've built huge and complex social institutions around making capital mobile, and even allowed them to suck up a rather notable percentage of GDP.
Imagine what a world would look like that put as much effort into the free movement of labor. And note: not all capital is internationally mobile, and in the same spirit, nobody would sensibly suggest that all labor would or should become mobile. There's plenty of room and benefits for investing both close to home (which is all part of "putting down roots"). It's a question of balance between local, regional, national and global for both capital and labor mobility.
It definitely makes it more difficult, but a lot of it is a "law of nature" because moving is expensive, time consuming, risky, and you give up your local connections. There's a reason for the metaphor "putting down roots."