Um, because the name sells, no matter what the input of the athlete is. BTW, the "input" of the athlete might not have anything to do with the way the shoe fits, and can be only stylistic.What's the point of a signature shoe if the player's input isn't a factor?
The above is wrong for all sorts of reasons. Have you thought about the fact that both KD and KB are/were sponsored by the same company, and releasing two different fits covers more foot types?I can't think of a single signature line in basketball (e.g. Jordan, Kobe, LeBron, KD, etc.) where the shoe is not constructed from a custom last based on the shape of an athlete's foot. Moreover, the signature shoes built from these lasts are the same ones that are sold to the public. How do I know this? KD's feet are on the very narrow side and his sneakers reflect that; many people with average width and wider do not find KD's signature shoes comfortable. On the other end of the spectrum, LeBron's sneakers have a wider base and heel, so people with narrow feet (like myself) find them too spacious and clunky.
There is no reason to believe that custom anything is the basis for a line of products. That is the definition of "custom", it is a one off, unique to the type of foot based on which it is made. It doesn't benefit anyone, who doesn't have that exact foot. The custom shoes of a number of tennis players differ from the shoes they are advertising, and rightly so.
A custom last is like every other last, so what you say doesn't make sense. It is not more difficult to make a shoe from one last than from another. Wawrinka has custom shoes from Yonex, and that is from before he was a Major winner, Nishikori and Dimitrov have custom shoes, and they were never Majors winner, Murray got a custom Under Armour shoes, and UA is a small company compared to the big brands (as is ON, for that matter), so clearly what I am saying has enough support in the facts.It's not trivial to build an athletic shoe around a custom last. Not only does the shape of the midsole have to match, but all the other pieces of the upper (heel counter, tongue, toe box, panels, etc.) needs to be the correct shape; there's a lot of prototyping and testing to get all the elements right. If you change the last, you will need to change all the other elements as well, so I'm very much in doubt towards your claim that "literally all top players of today get custom lasted shoes, if they ask for them."
Well, look for shoes for the aforementioned players. Comparisons between the custom shoes of those players and the retail versions have been made on these boards, and these comparisons show clear differences in the last as well as the construction of the shoe (materials, features etc).Also I'd like to see cited evidence for your claim that "custom lasts have been known in tennis for well over a decade." A custom last is not as simple as a custom orthotic.