You miss out on expandability for future needs.
I built a Windows desktop and it looks great, and it runs cool and quiet and it's efficient. Those were my design goals.
I don’t think the expandability is such a big deal. It only really matters if you try and upgrade a ten year old machine to run acceptably today, and even then such a machine comes with all sorts of compromises in the processor, bus speeds, peripherals and so on.
My experience is that a consumer machine rarely gets any upgrades. Apple will have done their research and realised that the vast majority of their customers would be fine with just a few options at the point of purchase. If it bothers you, get an M1 with the 16 GB ram upgrade.
As for Windows desktops looking great, well, perhaps your home built machine was an exception. Most of them are clunkers.