And yet the number of DDs keeps increasing (and Debian is one of most successful projects).
Indeed it takes time to do packaging, and this is by design. Packagers are expected to thoroughly review the code they are packaging and smooth out various sharp corners.
Many times I've found bugs in the upstream code while packaging. Sometimes around security and privacy, often around documentation, usability or non-x86 architectures.
Every time I check if other distributions opened bugs or applied patches for the same issues. It almost never happens.
This is why I might spend 2 hours on a package instead of 10 minutes.
Well, I do (sample of one). The Arch installation has been streamlined a lot. Now it’s all about
0) boot the image (cd, usb, PXE), which is actually an Arch install
a) creating your filesystem (pick you poison), and mounting it
b) telling pacman to install base, base-devel, and a bootloader on the target fs
c) installing and configuring the bootloader
d) rebooting
Done.
d-i barely takes care of that for you, it’s “just” wrapping it behind a UI (which is sort of useful, sure saves one from reading docs, but has been an annoying abstraction/obfuscation/magic layer for me more often than not).
The remainder (setting up X/Wayland/whatever is no different on Debian than on Arch, as d-i does not help much.
I can at least tell you that people don't choose Arch over Debian for the installation experience. :)