alt Hacker NewsStartup is the one thing that Arch Linux doesn't let you tweak, so I'm not surprised that they didn't manage to get a very fast startup time, which necessitates suspending instead of powering on and off.
I installed Artix Linux on my desktop computer, which is basically a branch of Arch Linux but with support for more initialization services, and it starts up a lot faster than my steam deck.
Nothing prevents them from tweaking the startup process, you're just making excuses. The reality is that they probably decided that most people will use sleep and not bother too much about cold starts.