Because I don't want the hassle of setting up and looking after my own e-shop, that's why. I don't have the time to administrate something like that. I have over 500 images on FP...uploading them all over again to my own site is not my idea of fun

Administration has never been my longsuit - I'd rather have someone else deal with it.
Paying FP 30% to handle the front end seems like a pretty good deal to me.
I don't think FP expects us to advertise for them - they do have a tiny advertising budget at this point tho - I would like to see them bump that up a little. Not all advertising costs and arm and a leg.
I advertise FP and promote my images there for a couple of reasons - I want them to succeed, because if they do, then so do I - also because like any endeavour you get out what you put in. I'd like to be able to sell as many images there as I do at other sites - I make almost as much there, not because I sell as many images, but because I can earn $7 from one sale.
I also hope that by trying to drag more buyers to FP, there will be more sales - sales all around, not just for me - why else would I advertise other people's images? Why? Because more overall sales means more earnings for FP, the more money they make (even though they still collect only 30%) the more available funding they have - money they can put into advertising.
And yes, I think they need to do more advertising - not necessarily on the internet, but in trade magazines, and at trade shows - canvas the design houses with personal visits (local ones anyway), and target producers like greeting card companies, calendar companies, advertising agencies and so on. But I really have no idea how much advertising they do.
I think they have a good business model, and the fact that they cater as much to their photographers as they do to their buyers is a plus from my point of view.
Let's take snapvillage - I had a look there, but they only will pay by paypal. That's a no go for me.
I finally bit the bullet and tried to sign up for a paypal business account in order to sign up at SV. Simple, right?
Except that I reside in Canada, I'm a Canadian citizen, and my business bank account and credit card are in US funds. Why? because 90% of the internet business is paid in US funds. Paypal won't send funds to a canadian bank in US funds even if the bank account is a US account. I have to have a bank account in Canadian funds. That means I'd be required to set up a new business bank account - no cost to set it up, but there are certainly monthly fees that go along with that account, and I refuse to use my family/personal account on the internet. To me, that's not helpful - SV should have other options. They are a new site, so why not offer multiple ways, like payment by cheque, direct bank transfer, and moneybookers, as well as paypal?
At FP, when you have a problem or an issue and you contact them by mail, it is dealt with pretty quickly - you are not ignored, nor sent canned emails, you don't wait weeks to get an answer or see a 'fix'. If you have a good suggestion, it's generally implemented as quickly as possible. Why do you think they started selling vectors
Unlike Karin, who has to make her living with her sales, I don't have to spread my stuff out all over the internet and take what I get (if I were forced to make this my primary method of earnings, you can bet I'd be doing just what Karin does). When you turn this business into your bread and butter it becomes a different matter - sales and earnings become paramount. Folks like Karin have to bring in the most money they can and so have to spend their time uploading at sites that are doing that for them. Time uploading at a place where they earn very little on a regular basis is time that isn't well spent.
For me, my business is already supported (minimimally, but I don't have to be the breadwinner, so my business supports itself) outside of internet sales. That means I don't have to make microstock income my first consideration in choosing where and how I want to market my work. That can make a huge difference in how you look at the various sites.
My focus is in creating an internet presence for my work that is something more than microstock - a place where they can purchase my artprints, my illustrations, or what I call "my stocky stuff". All in one place - FP can fill that better than most. Yes, shutterstock sells those things too, however, I still shudder everytime one of my artpieces sells for 25 or 30 cents.
I guess I am looking at building a future, rather than earning my daily living.