As a working father with kids I exist on so little sleep the idea of having breakfast, much less making some for my neighbour is equally inconceivable.

I barely get a moment to drink my room temperature coffee while I pack their things for school.

Chandler has been best friend with Ross since college, and he lives across the hall from Ross' sister and he's close friends with her too, so it's not that weird that he comes over regularly given that they all hang out together all the time anyway. Specifically about breakfast, the girls also complain sometimes about the boys "stealing from their fridge" every day. And then there's Ross, who is the brother, actual family, so it's really not weird for him to eat with them if he happens to be on the way (although we don't know where he lives in the first half of the show). He seems to do it less frequently than Chandler and Joey, too. Phoebe is the one who is the least close with the rest, just a former roommate, and appropriately, she doesn't raid Monica's fridge every morning.

Maybe it's not the norm, but it shouldn't be that weird, the circumstances make it plausible. It's the whole point of the show being about a tight group of friends and family. If anything, I would imagine the weirder part is that they all keep their door unlocked almost all day long for anyone to raid the fridge, but that's the part where it's just "it's TV, don't overthink it."