Because the user experience sucks. I hate copying a URL and have a bunch of meaningless garbage included in wherever I paste it, as happens with pages that leaves UTM artifacts in the query string.
I don't care at all about the tracking aspect of removing UTM, I care about how it makes the internet shittier to use.
I don't care at all about the tracking aspect of removing UTM, I care about how it makes the internet shittier to use.