All of the listed features significantly raise the bar for exploitation ;
Why can't the stock ROMs use these features and be more secure also?
I had to get my friend to buy them for me when I was on Graphene
Some of the features may hurt user experience in some way and people made different trade-off.
For example, GrapheneOS disables USB before unlock so that there's no chance that some driver codes in Linux kernel run in response to a device being plugged in, for attack surface reduction. Then, say, if you have a cracked screen, the touchscreen no longer works and you don't want to fix it, if not for this mitigation, you can use an USB-C OTG cable to connect a mouse / keyboard to the phone, unlock it and export all your data. With this mitigation the keyboard won't work so you are forced to fix the screen first just to get your data out.
Some of these features are backported to mainline android, others may be deemed too advanced or just the incentives don't match (e.g. being able to disable networking by the user could cut into Google's earnings, e.g. limited ads in apps).
I guess one reason you'd want to avoid that is that makes it harder to e.g spoof your location or falsely tell the app that screenshotting is disabled.
If you want to dive deeper you can checkout droidguard/play integrity.
It also runs on Lineage with Mind The Gapps.