Yeah, it could be that many startups try to do advertising ethically, realize that it doesn't work, and drop out. That's necessarily going to leave all the firm

Yeah, it could be that many startups try to do advertising ethically, realize that it doesn't work, and drop out. That's necessarily going to leave all the firms willing to sell snake oil behind. You can get firms selling stuff that doesn't work that are "trusted" simply because they've been around for years; there are plenty of distinguished brands selling homeopathic remedies, audiophile speaker cables, timeshares and MLM schemes.