Why most PTC sites turn into scam?

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As we all know all PTC sites don't pay us from their own account, They pay us for our work done for their advertisers. and they get more than $0.1 for per click. Most sites offer us just 1-2 cents. Still they are just scams. Sites like Neobux,buxP, clixsense pay $0.001 maximum. Why is so big difference? Do they earn so big amount through our clicks?