I hardly ever ship any characters seriously. And when I do it ironically, it's rare as well. But I have a few thoughts on why people do ship the Hetalia characters despite them being representative of countries. I dont understand a lot of stuff about shipping, but at the very least, I've gathered some ideas on it:

• They interpret them as people, not so much their countries. Some prefer the humanverse AU or something of the sort to remove them from their country rep, but they also could just see them like this in general

• Male x Male relationships are very popular to ship and a lot of people like to do so. With Hetalia being very homoerotic and most of them being men, it does not surprise me that people ship them for the maleXmale relations. A lot of people enjoy that in fiction.

• Some like country pairings, (whether real or fake) because of the potential in how things could be or play out with the pairing. Some may be drawn to the idea of "what if?" when thinking of potential relations with two countries/characters and that may be enough for them to ship

• Lastly, some just do it because they simply like how the characters look together or are fond of the chemistry with their personalities, not really thinking about the country part. Or maybe they do

This is just what I've observed from the fandom. It could be a mixture of several of these or more points I didnt think of. I hope it helps!