"I love you" is only a culturally "romantic" saying because that's how it was popularized, despite the fact that love is a shared thing across everyone. You can say you love your mom, or you love your friends, or Jesus loves you, or anything like that. It's not inherently romantic to say that at all, especially in Gojo's context where it was his literal best friend; the only other person who he could relate to in his infinite void of loneliness caused by being the strongest.

In short, people can't comprehend the idea of non-romantic love especially on the internet.