The point is, that Itch might change their tos and do keep frozen versions of a game for access, similar to Steam. But in their current tos they do not act in such a capacity and push all privileges and duties to the publisher. The privilege is to be able to remove the content. The duty is to not being able to revoke the license and it would be very wise of a developer to remove content in a way that does not result in eglible refund requests. Removing content is rather unwise to begin with. But making people not act unwise by tos is practially impossible.
To speculate about reasons: there is no curation of the files. The publisher handles all of them. There is no platform integration. It is just file hosting. Steam games on the other hand are integrated into the platform. The developer cannot just change files or functionality on a whim.
The plus is, that drm is not a thing on Itch in most cases. The minus is, that you should always download your purchases. Including any mega bundles.