You can build Debian packages as Arch but that wouldn't be the Debian way.
Stability - our QA process is huge and much more strict then the Arch one. We often notice upstream bugs during our packaging and testing process and we also often patch them before upstream.
Security - we have Debian Security Team which is ruthless and security is a big focus so when for example the Chromium security problems was found, it was also noticed that that feature was disabled in Debian chromium package by default.
Upgrading - I don't think any distro can offer this without issues and those issues are exactly prevented with out packaging guides and Debian Policy. In Arch you would get from time to time - please do this manual things in order to upgrade to next version of this software. While in Debian you can go to trip to Mars, live there for few years, come back and then just apt update && apt dist-upgrade the thing and go on coffee without worry.
Also notice we had much before Arch the gpg signing of packages, you can't become Debian maintainer without at least two Debian Developer signatures on your key which happen in face-to-face meetings (so we create a web of trust you will not introduce bad code in your packaging).
So yes, you are correct that easier is to create Arch package then Debian official one, but in Debian you can also create very quickly packages (checkinstall, fpm etc etc) but then you miss all the power of Debian and you're on your own with those packages and potential breakage with system (or is that a freedom you have?) :)