Most of us can make a stock frame work with a few adjustments, but some riders have physical proportions that simply defy mass market sizing. When you have an extreme leg to torso ratio, you often end up with excessive handlebar drop or a cramped reach that no amount of stem swapping can truly fix.
In those cases, investing in a bespoke custom built steel, titanium, or carbon frame becomes the only viable solution to get the geometry right. Beyond just the dimensions, a custom build allows for things like tube butting and wall thicknesses tuned specifically for a rider's weight, which is a level of precision you won't find in a production catalog.
At what point does a rider's proportion gap make a bespoke bicycle the only real option for a proper fit?