It is the dynamics. When lanes are pacing each other the gaps all tighten up. So sure, you can slow down a bit to find the gap behind you, except that gap is not big enough to fit in. So you turn on your signal and wait for someone polite enough to let you in, meanwhile the guy behind you is riding you like a pony because you are no longer keeping up with traffic.

When traffic isn't balled up so tight, you can plan for a lane change in advance and accomplish it without having to slow down traffic. Everything flows better.