wondering about PTC

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ever since i saw PTC sites be it a reputed site like neobux or scam sites like bux.to ,I have noticed only one thing. All PTC sites just have ads showing other PTC sites or sites like these. So if they all are advertising on each other where the money is going to come from because the ad on each other would cancel the money earned for one site. No PTC site has any actual advertisements. So you dont see a Gillette ad or a Nike ad. No wonder nearly all PTC sites go scam sooner than later. What are your thoughts on this?