Yeah it comes down to "over-use". Any popular area just sees so much use these days, with the huge population of people.
Same reason you have to pack your poop with you on overnight river float trips. There's only so much space between the cliff walls along the river. Typically there's someone in every campsite, many if not most days in the summer. That's a lot of poop. Even two people peeing in the same place, day after day all summer long, and man that's gross.
Unfortunately, someday that's going to be the reality on backpacking trips, too. Carrying your poop on your back. I hate the idea, but I also hate showing up to a campsite and not being able to walk any direction without running into toilet paper and worse.
When I'm in the desert, I use my solar
shower, outside, and let the water go onto the sand. I avoid areas that see a lot of use, and no one is going to notice 3 gallons of water with a little biodegradable soap, typically just once in any spot I stay, or if I stay long enough, once every 3 days, probably. Someday I won't feel ok about that anymore, as those areas see more use.
Outside showering is great. You don't need to be naked to shower. Sure it might get slightly awkward when you go under your shorts to do some scrubbing down there, but no one can see anything they shouldn't, and it's easy to look away.
I agree with some other posts: if I had to choose one or the other, I'd choose inside shower. But both would be great. I might rig something on my camper at some point. But until then I have a solar
shower in case I want an outdoor shower.