Often larger sites have the sitemaps on a different subdomain and/or are setting up individual GSCs for the subfolders. Google even says this has to be done to be able to pull more than 50,000 rows from the GSC API. See https://developers.google.com/search/blog/2022/10/performance-data-deep-dive

Examples:

root domain: https://www.yourdomain.com (which may have a bunch of users)

main sitemap index: https://sitemap.yourdomain.com/sitemap.xml

example of subfolder set up as unique GSC property https://www.yourdomain.com/sub1/

example of sitemap for subfolder, contained in the main sitemap index: https://sitemap.yourdomain.com/sitemap-sub1.xml

Following Google's suggestion to set up these URL Prefix GSC Property Types, does not allow the site owner to add sitemaps to the GSC property because the sitemap (in the example above) is on a different URL prefix, so there is no way to add it to GSC for URL monitor to auto pull from there.

URLmonitor is great but adding 20+ users with owner permissions will be scary to some people. Generally, larger sites as setup above only have a few SEO people in the individual subfolder GSC properties. Also, these larger sites may wish to use these subfolders to direct the pages that URLmonitor checks and indexes and those it does not.

An alternative to this would be the ability for users to upload more than 100 URLS manually or with a file upload.