You may have a point for some individual proponents, or snake oil salesmen as I would call them, but not all. Particularly here, I'm confident that there are several proponents or, like me, cautiously skeptical optimists, that are involved out of a serious desire to provide an answer to this question.
If information over hundreds of reports (height, sightings by season, footprint size, etc.) has a bell-shaped distribution, which is the easier explanation - that people are reporting an animal that varies in a natural distribution like other animals or that hundreds of faked or miss-identified encounters were managed to ensure that they created a natural distribution. While I agree that the ultimate proof of existence of Bigfoot won't be proven w/out a body, I'd like to think that intelligent inquirers looking at information intelligently can glean valid information from reports.
That is the rubik's cube that hooked me after a solid 30 years of not thinking about this.
It might not be a mystery to you, but it's an open question to me. And I participate - in a quite time-consuming way now that I have the time to do so - in an effort to answer that question to my satisfaction. My answer might not change your mind but my goal isn't to change anyone's mind but my own.
Again, I certainly think that certain people and groups have more of a business interest than an actual interest in actually finding anything. But just like other pursuits - religion and political leadership come to mind - the fact that some charlatans take advantage of the situation doesn't detract from those who try to faithfully carry out their mission. I guess the argument I would pick is that your argument sweeps too broadly.