Decentralization is as much about who is administrating than it is about where and how it is hosted.
We centralize hosting, but not administration. Some centralization has very valuable perks, like making it much easier for anyone (not only experienced sysadmins) to own a Mastodon instance and participate in the federation, or pooling up resources to share a powerful infrastructure even with small instances. But it becomes a single point of failure: more instances will risk having issues at the same time, and a few people get more responsibility and power over the network.
We believe our service can be very useful and can help many instances exist in good conditions, or exist at all. But Fedi.Monster doesn't aspire to host all Mastodon instances, we only want to make having a Mastodon instance easier and available to more people, especially those who can't self-host for any reason.