Why the co. names?

...I just looking at a Falcon motorcycles picture and I really do not cut for me this thing about "a company name motorcycles"
Do they make the engines?...NO
Do they make % of the rest of the bike? yes
Do they use some parts of other brands, like wheels, drums, brakes, forks, etc? YES

Here in this forum we see that there are people who live from building motorcycles or repair them and others like me that are building one
but I do not see "my name" motorcycles...I see my Triumph bobber, HD pan build, etc
It s the engine the most important thing and it s the engine who does the name to the motorcycle.

so , why a bike that have a Vincent, Triumph, HD, or whatever and never be an agreement with those factories, etc is called "a name" motorcycles?

Im wondering about it