Well, with packages you want various filtering steps to happen before they make it into users' systems. Or layers of security that make it harder for a system to be compromised.
Let's take a look at the xz incident, then at how fast rolling release distros get their packages in. That's part of the equation. Bottom line is: you're the first line of defense against potential malicious supply chain attacks. This is why Fedora is Red Hat's testing distro, why Debian has an unstable branch or why openSUSE Tumbleweed exists. Now, Arch isn't just a "testing distro", but it is, by design, more susceptible to these attacks. Thinking bleeding edge is more secure is a fallacy. It is but a consequence of assuming the source maintainers are on your side, which is usually the case, but not always. Or, assuming software is properly tested for bugs every release. If you are still doubting this, look at npm.
Furthermore, have you ever asked why you need to constantly update package signing keys? There is no central build server for Arch. Maintainers are building packages in whatever machine they are on, signing with whatever keys they have there and uploading the binary blobs. This isn't trustworthy. There is now a clean chroot process and all, but still, maintainers are still able to build the packages in their machine and upload it.
The other problem is not having any mandatory access control security policy by default (SELinux, AppArmor, etc...). You can, of course, install your own and go through the trouble of actually creating the security profiles yourself for the various packages on the system. This is in stark contrast with other distros, where not only they provide a security policy by default but their packages also ship with security profiles when needed to make sure it actually works (Fedora and openSUSE come to mind).
Finally, the AUR is cool and all, but my god are you at the mercy of whatever is put on there. Sure the PKGBUILD is super legible, but are you really checking where things are being pulled from? There is a layer of filtering being taken away here, you are the one doing your due diligence.
Now I'm sure different people have different takes on this, some might say that security policies are dumb and useless, others might prefer to be in the bleeding edge assuming the latest and greatest is safer. But take all the layers I have mentioned here, and how their non-existence on Arch could affect security. I hope to have drawn a clearer picture.
(Edit: I must say, I like Arch, I've used it a lot and when in a pinch is my go to. But I've come to appreciate how other distros approach security, and how they layer the process so they have more time to assess vulnerabilities. It is a balancing game, and I hope Arch improves on their processes, I really do.)