The reason you don't see it is because it is very, very difficult.
Basically, they will let you start at the exit so you have a few extra seconds to try to make the transfer, and they can slightly slow the boats - but it entirely depends on if the person with a disability can walk a few steps or not and how agile they are. Someone that has to be manually transferred (a family member has to assist them because they cannot walk) is nearly impossible, as is it for anyone who manually transfers themselves (like one does on most rides, like transferring to a car seat and back from a wheelchair).
My niece has CP and cannot walk, and when she was younger we were able to just pick her up, run along side, and carry her on. Now that she is an adult, it is just impossible. They can't just offer to stop it (see above post), and can only do so if there is a fall/etc. Even when we could it was incredibly stressful, this is easily the most difficult ride to board at WDW.
Pan and TTA are the two non-accessible rides at MK that one wouldn't expect. With TTA, they simply won't let a wheelchair up there to even try. It's a shame - that for the most gentle, relaxing ride at WDW (TTA), and a ride where everyone gets to see what it would be like "if you could fly" - that you can only board if you can walk.
