There are many companies offering repositories. Are there companies with S3 like billing? Basically, where we are billed based on storage/bandwidth/requests?
To some extent you'd be penalizing people for success.
"Your project blew up, pay us thousands of dollars" is a non-starter for a lot of places, and optimizing around small consumers isn't generally to a business's interest here.
looking around at other services: Gitlab won't add it to gitlab.com hosted repositories, nor does bitbucket seem to offer auto-magically rescaling + on-demand. https://gitlab.com/gitlab-org/gitlab-ee/issues/3314 https://bitbucket.org/product/pricing?tab=host-in-the-cloud
However you could run gitlab in an automatically scaling AWS package (or google cloud, azure, any other service that offers elastic packages).