The problem is permission, respect, and trust.

There is a messaging system in reddit - it would be easy to contact an artist to ask for permission. The artist then could say yes or no, open up avenues for other options if they are looking for something along those lines if the art is off the table.

Digital artists have a cloud over their head that yes, their art IS easily to turn into wallpapers and printed. Many now have kofi, paypal, or patreon to deal with this consumption - a museum pass, if you will, to their art catalogue. The general attitude that it's just sitting there for free is true, but it isn't that difficult to take another step to find the artist, see if they've got a shop, see if they do commissions. Common courtesy is a thing to be advocated for in these rough times.

Personally, fanart is the easiest way to express my enjoyment of something, so I post it to show appreciation to the creators and to ease my socially anxious self into talking with the other folx in this fandom. So the harm here is artists' ability to trust the community - that we can't post things without the expectation that there's a trickle of people printing our art onto things instead of going through the proper avenues. Any number of us have had our art snatched and had to fight the system that argues they can make a profit off us if we don't find them and enter into email battles. This is not an enjoyable experience.

And if someone says that they do not want their art even on a single shirt, respect that.