Why do people seem to have this tendency to only say "I love you" when it's too late?

Answer 1

because they are afraid that they could get hurt when they open their heart. simple as that.
on the other hand, people sometimes are afraid when they hear those words too early, because they are not sure about their own feelings yet and think they are forced to answer: "i love you too."
that's why love is so amazingly different - because you never know for sure.