>The problem with early merges is that there's no way people that merge in are at highway speeds
If you drive anything reasonably modern and not grossly overloaded/under-powered you can plant your right foot and be going traffic speed (or close to it) by the end of all but the smallest on-ramps. Most traffic is not made up of box trucks and panel vans so there is no technical limitation that prevents zipper merging on most on-ramps most of the time. Going close to traffic speed makes it a lot easier to merge but a lot of people aren't comfortable doing it that way.
If you drive anything reasonably modern and not grossly overloaded/under-powered you can plant your right foot and be going traffic speed (or close to it) by the end of all but the smallest on-ramps. Most traffic is not made up of box trucks and panel vans so there is no technical limitation that prevents zipper merging on most on-ramps most of the time. Going close to traffic speed makes it a lot easier to merge but a lot of people aren't comfortable doing it that way.