it is agile and easy to move in, you're a perfectly capable and superhumanly tough robot man
it's just that i, the player, prefer being on foot and being relatively small then stomping around in a big shiny 'shoot me' sign even though the big shiny shoot me sign actually has a bunch of reasons to ride in it
also the 1000 year fusion core thing is definitely just a plot device to facilitate the player wearing power armor like it's nothing without maintaining it; if anything, the new fusion core system makes way more sense to me because something as powerful as power armor doesn't feel powerful unless even wearing it is a deliberate choice you have to make and an expenditure of some resource
and beyond that the express purpose behind power armor is that it's the fallout universe's equivalent to tanks; extremely tough and can easily shrug off small arms fire and, as an added bonus, extremely mobile and capable of wielding larger ordnance