My daily driver Lumina had an internal electrical failure in the tranny that sent it into limp mode. 2nd gear starts (with the selector in 1 or 2) and manual shifts to 3rd. I drove it around for a couple weeks before I swapped the tranny out, joking that I had a race Glide in it.

I guess since most race cars have manual valve bodies anyway one could build a TF and start in second gear if they really wanted a Glide.

The true answer is probably (without knowing anything about whats out there) that there is much more aftermarket stuff available at a reasonable price for the Glide, and obviously having less gears makes the construction simpler, allowing for more durability. I'd bet its aftermarket though, same reason a 350 is the greatest engine ever and a Ford 9 inch is bulletproof.