Not everyone cares for sex, but for many people romance involves sex and intimacy. That’s not to say your lover can’t be your friend, but platonic is specifically about “just being friends.”

I think there’s also a vulnerability aspect to it. Many people let down their guard around friends, but I imagine around their lover there’s a lot less walls up. You might meow and pretend a cat to wake her up, or dance stupidly together in your living room. But when you’re with friends, no matter how close you may be, you might not show that side of you to them ever.

Also I don’t get a boner around my friends (intentionally), no matter how attractive they might be. But it becomes hard to walk just by holding hands with someone I like romantically.