I guess there's another reason too. You don't need to give personal info to all of the companies. They don't get your address, phone number, name, etc. All you have to do is view the page for a set amount of time. No forms required (except to sign up for the program).
Because one main reason; it's free! It's great for us new people into the whole network marketing business. Especially when you don't have a creditcard and can't pay anything to join a site cause you don't have any money on your paypal/alertpay.
I'm a member of PTC and about to cashout almost $17 which is OK, but the main thing is that I can now buy myself into other sites that requires payment.
And also, those surveysprogram are usually only for US-Citizens so.. :P
But GPT sites have lots of free offers and other ways to make money. And even if there are not a lot of offers, some have very generous referral programs.