Because they didn’t do what you personally wanted, you think they didn’t put thought into it?
Looking at the magnetic connection of the power cord, the extremely thin profile, the inclusion of ethernet right into the power brick, you’d have to say they put a massive amount of thought and effort into it.
Sorry you don’t like it, but this wasn’t something that was cobbled together without a thought.
The fact they dressed it up nicely doesn't negate the larger point. It's an
external necessary component on an supposed all-in-one desktop computer.
The power supply has been internal since the very first model, over two decades and 8 generations ago. Heck even the original Macintosh had an internal power supply in 1984! Was Apple and the countless designers over that entire period of time wrong then, or are they wrong today? Nothing about the decision about internal/external power supplies has changed since then. And if anything, Apple has been making more things built-in and internal over the years, so this move is a clear outlier. Even the AppleTV has an internal power supply! This is a regression, and nobody has credibly argued otherwise so far.
It's also not like an ethernet or display adapter that can be dongle-ized. It's a necessary component without which the product cannot be used. This makes it no longer an all-in-one. When does an all-in-one stop being an all-in-one? How many components are allowed to be external before it stops being an all-in-one?
What does it say about Apple's own confidence in the power supply? Power supplies are external when they need to be for portability (e.g., laptops, tablets, smartphones) and when they are cheap and likely to fail (and thus designed to be easily replaced). The iMac is certainly not a portable device. So is Apple expecting these things to fail and need frequent replacement?
Besides just giving Apple the benefit of the doubt, nobody has actually put forth any reason the power supply had to be external on the iMac, nor why it should not be internal on the Mac Mini. The M1 Mac Mini uses just 39W of power at peak. There are 65W power supplies that are approximately 1 inch cubed, and some designs are under 1cm thick. It can easily be made internal to a redesigned Mac Mini, even if the goal is to make it thinner and sleeker.
There is objectively no technical reason it cannot be integrated, and no benefit to having it external (unless Apple is not confident in their quality and reliability).