It also displays the extreme nature of Phil's ignorance and inexperience with the most odd things, because I don't know how you make it to the age of 40 and not get used to the weight of a milk jug, to the point where it doesn't occur to you how heavy it is. I mean, who thinks about the weight of their shoes, ever? How many grown-ass women consider the weight of their purses until they accumulate too much in them? How many college students think about the weight of their backpacks until the same thing happens there?
Wouldn't be nearly as bad if Phil would go out of his way to fix the problem, but instead he acts like he doesn't have one, probably from literally not knowing it, since he's so stupid. His IQ really fucks him out of improving in any aspect of his life, and is more than likely going to be the end of him one day, given his current condition. It's not like he's ever suddenly going to take life seriously for once and stop being lazy, after all, so whatever his trajectory is now, the end point of it is a foregone conclusion.