Physicality and Rejia have the highest energy concentrations; Rejia because it's at the top and has Lysial actively condensing energy there, and Physicality has its energy cycle which temporarily traps some energy as it cascades down. Also because it's the largest plane and has the most divine activity.
Rejia is basically impossible to breach and its denizens- Angels- Tend to view those of lower planes as, well. Lesser. Usually in a sort of pitying manner, like 'oh, those poor beings, so far from Lysial's grace'. They also have a particular disdain for Malachar and Infernians. Malachar, because of the actions of their god, which the Malachar don't really give a flying fuck about, and Infernians because they keep trying to get into Rejia. This is the equivalent of a guy who's made it his life mission to throw rocks at your window from across your fence. Only rarely does he actually manage to ping a pebble off your nice sturdy window, most of the time his rocks just land in the yard, but you do kind of worry he's going to get a good shot in one of these days and it'll actually break the glass.
Infernians are the only ones who have been at all successful in traversing the other planes, and even then they're not that successful, and it is specifically because they feed off the energy of other planes, so there isn't as much energy friction. Also, they're basically the only ones that have made a concerted effort, since the dawn of their existence.
The denizens of The Wild Sea are basically too busy partying to give a fuck about anyone else, though they do love a good shenanigan. Malachar is a bit too hot for their tastes, and while they'd absolutely love to party in Rejia, there is the aforementioned problem of it being impossible to breach.
So, they tend to make this Physicality's problem.
Also, many of the interplanar instances in Physicality are rarely recognized as such to the mortals that live there. Fey are fey, demons are demons, and it doesn't matter where they come from. The distinctions are basically only recognized in some scholarly circles (such as Auhaelan Academy, and even then, there's very little official knowledge or understanding of things such as demons)