The "FOSS" alternative is a single model with their own site, which some bigger ones have.
Even then, they have to setup some payment processor, on top of domain/hosing costs and find some solution on designing the actual page. Mostly doable, but a pain. Except I don't think we'll have FOSS payment processing software until (if ever) cryptocurrency becomes mainstream.
This is like asking why everyone uploads to Steam when they can just publish their game on their own website. These services are selling convivence and curation (and to a much lesser extent, "security". i.e. the website isn't full of viruses), like 90% of the big websites.
Even then, they have to setup some payment processor, on top of domain/hosing costs and find some solution on designing the actual page. Mostly doable, but a pain. Except I don't think we'll have FOSS payment processing software until (if ever) cryptocurrency becomes mainstream.
This is like asking why everyone uploads to Steam when they can just publish their game on their own website. These services are selling convivence and curation (and to a much lesser extent, "security". i.e. the website isn't full of viruses), like 90% of the big websites.