If the designers of the phone want to provide a reasonably priced cover that will hold my credit cards and protect the screen then perhaps they can make one that complements the device; and perhaps I'll buy it. Until then I'll have to make do with what the third party suppliers can deliver, there is no way I'm going to carry my mobile without having it in a case.
Not everyone prioritizes buying thin phones, but it appears aesthetics is a big deal for you. I'm your opposite: all I want to see when I'm using my phone is its screen. Having a case made out of soft material that can absorb energy when it is dropped (not "if") has saved it on multiple occasions at the cost of adding ~1mm to the thickness.
Stickers are a way of expression. Sometimes, I don't like the idea being expressed, sometimes I do. Clean is fine too.
Phone covers are usually for protection. And a reason I hate phones with a glass back (thankfully a dying trend). It is fragile, slippery, heavy and not structural, it only looks good in a store because people put on a case afterwards. Pre Galaxy S6, Samsung had the perfect back cover, made of light plastic, openable and replaceable, and in the case of the S5, waterproof. You may find it ugly, but under a protective case, you don't see it. In fact, they even sold cases that replaced the back cover.
I have stickers from community projects that I actively contribute to or projects that are really meaningful to me and I support morally and financially.
In limited amount and size not to be tacky...
Many people apply the same logic to tattoos.