[jstanley]

If you don't leave a gap then you're forcing the people in the other lane to slow down and disrupt the flow of traffic.

It is a mistake to think that one of the two input lanes is more entitled to the output lane than the other. 2 lanes go down to 1, neither is more privileged than the other, cars should merge in turn.

[practicemaths]

If there isn't a gap then don't force a merge. Otherwise you're the one driving unsafely and causing further traffic congestion.

Your comment makes less sense when the merging lane has no one else it as well. That means the car in it that is merging over has plenty of opportunities to merge safely without causing traffic congestion however they don't.