Popular media has made it such that some people do call other stars ‘sun’ though.

e.g

Krypton’s red sun

tattoine’s twins suns

Romulan sun

Etcetc

So much so that I think many people would imagine standing on an alien planet, looking up at their version of the star the planet is orbiting, and casually say ‘damn the sun is sure hot today’.

Edit : I’m not saying people are calling every star as ‘suns’ interchangeably, I’m saying people call the star a planet is orbiting as the ‘sun’ of that planet.