Because the hosting costs are the cheapest part of it? The real, time-consuming work is building the site up front, then on an ongoing and perpetual basis: doi

Because the hosting costs are the cheapest part of it? The real, time-consuming work is building the site up front, then on an ongoing and perpetual basis: doing technical maintenance and updates, moderating the site for trolls and spam, monitoring and logs, backups, user support, community administration, etc.. It's also thankless work because wikis, unlike forums, are much harder places to build a critical mass of users who will be engaged and want to invest time and effort. So you have a proportionally small number of people who maintain the content, and a potentially large audience of read-only drop-in users who are not invested in the community in any way, but just want answers so they can leave. Finding people to operate this kind of site reliably and long-term, unpaid, is really difficult. So instead, for better or worse people get it for free from Fandom, in exchange for ads.