Because it's pronounced the same. It's internet slang.

There is no such word in English as "luv".

due to the pronounciation and it's only used in chat

The word love is used in many different ways she loves ice cream she loves her husband she loves going to the park she loves The Rolling Stones, Lady Gaga and Real Housewives of New York

L means sound [l] u means sound [ Λ ] v means sound [ v ] So.. It's love :) Why - It's a slang, It's simpler to write and looks like something unusual and cool