It’s because it’s not standardized and every watchmaker differentiate the case one another. Hence, it’s better to get one from the manufacturer or 3D print it by yourself.

Bracelets are sold by lug width, not lug-to-lug length. The length of the watch is irrelevant. Bracelets with curved end links are generally listed as fitting a particular watch. Watch brands can’t practically manufacture and sell end links that fit every aftermarket bracelet. Let the aftermarket companies sort that out.

Bracelets with fitted end links are watch specific in a lot of ways, there are too many variables to make generic bracelets practical. You just can’t buy a bracelet that wasn’t made for the specific watch and expect it to fit.

If there is an option to buy a watch with a bracelet, you should do that. If there isn’t, you should assume you can’t get a bracelet with fitted endlinks for that watch unless it’s one of the really popular models that gets third party model specific support.

Because every watch is different, there is no standard set of fitted end lugs - you need to get them specific to your watch. Not only would you need the width and radius of the curve of the endlink to match, but also the height, the shape of the back where it sits against the case, the shape of the front to match the lug design, the finishes and more.

Because of this you just don’t get aftermarket bracelets with fitted end links for most watches - some brands may have them available as an option, and there are a few third party options available for some very popular watch models like the Seiko SKX, but if the manufacturer doesn’t make one, you are out of luck for most brands.

What is available as an option would be a bracelet with a straight end link - this means as long as you get a bracelet with the correct lug width (usually 18/20/22mm) it will fit, but there will be gaps between the bracelet and case.

Who sells bracelets by lug to lug dimensions? First time I’ve ever heard that.