My guess is two part.
One, while we know the power they can produce, many are still ignorant and if presented with the need for use of transmissions with higher torque capability, most in the business of making such adapters would simply ignore it and possibly think that you are nuts.
Two, like most Y parts, there just isn't the market for it to justify the costs involved when a company may already offer plenty of other products that are paying the bills.
Things like that will generally have to come from enthusiasts, but will most likely be custom solutions.
I think the primary reason that a T-5 is an option is simply that the same 3 speed transmission pattern was used for quite a while and was in use on mainstream engines long enough that there is a good market for the 3 speed to T5 adapter. While I am aware that Y-specific adapters for 3 speed to T5 have been made, those were niche products / solutions and not something that you can get from almost any catalog.