Aesthetics can be very important because attractive things work better:

https://www.researchgate.net/publication/202165712_Emotion_D...

Apple could have found a way to put the button somewhere else and make it nearly invisible, but that's expensive and the Mac mini is clearly designed with cost in mind.

If you want cheap and functional, you're in luck because that's pretty much all anybody makes.

Apple makes it difficult to access because they want to make sure you don't use it often, as they believe the experience of waking up the computer from sleep is better than starting it up.

It's a conscious decision based not on design, but on UX, as with the Magic Mouse USB port.