Why PTC Sites Become Scam?

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i am wondering why many ptc sites turn into scam. they say that the site losses. how come they loss money while advertising to them is not free. every ads we click are paid by their advertisers. did they allowed collectible advertising? my opinion is that, they used the site to earn more ,so if they earn already to what they wanted, automatically they shut down and system and gone. any idea are welcome specially owners of ptc sites.