With the literally millions of websites out there trying to sell something, it's very easy for an offer to get lost and never be seen by anyone. So, they advertise and as we all know, advertising costs money. Almost all Get Paid To... programs are splitting the fees received from the advertisers with their members. Also, since the more members a program has actively working it, the more attractive it will be to advertisers, there's another market just to advertise the PTS programs themselves and this also pays us.
There are a few different types of paid program out now. Things really started with Surfbars, where a small bar ran along the top or bottom of your screen, displaying various adverts, so long as you were connected to the Internet. These have pretty much all vanished now, replaced by more direct methods of getting potential customers to see what's on offer.
"Ah, but folks are getting paid. There are testimonials to say so." This is true, folks are earning money, the people who set the program up and those who joined very early (my best guess would be first day of launch) will make money, after that getting your initial payment back would be close to a miracle is my thinking, as every time someone makes a profit, someone else missed out on nearly breaking even. To add to the fun, most of these programs can very easily be classified as "pyramid" schemes which are illegal in most of the First World.
Now, I'm not accusing anyone here of scamming, or any other such mischief. I'm sure that any number of these 'opportunities' are started by honest folks, it's just that the very model the programs use mean that a lot of folks (the more successful the program, the higher this number) are going to be left out of pocket.