The front end is designed to flex and give you a better ride. The front wheel absorbs the bumps. The handle bars and grips have little to no vibration. It's like a built-in rubber mount. It's all about basic engineering.
Unsprung weight is a big factor in motorcycles.
A girder front end has more than a springer but THE LEAF has the most flex of all. It only works on rigid frames because it pivots on the rear wheel, but you can use struts for other types of frames. THE LEAF was designed for extended front ends only. They do not work straight up and down like fork tubes, just as fork tubes do not work properly on extended front ends. Tested on a Big Twin with a 240 lb.