In the olden days trailers were an option, not part of a "package" put together by the manufacturers. As such, boat dealers set up the rig so it was right, and would last the owner a long time. Back them ALL trailers had keel rollers.
Although I don't go that far back (oldest boat I owned was a '73), not a single one of them had keel rollers that were meant to support the weight of the boat. I had a couple trailers (3 that I can think of) that had rollers at the back, but they were meant to guide &/or keep the hull from hitting a crossmember while loading. Once it was on the trailer, the roller was well clear of the hull. And on those trailers, having the roller too high made the thing nearly impossible to load, as it would roll off to one side or the other without the bunks self-centering it.
I've honestly never seen a bunk trailer that had keel rollers that were meant to be supporting part of the hull. Sure they exist, just have never seen or owned one.