Though a lot of these errors are in the package metadata, Debian doesn't make it easy for itself as the build system and packaging format is very complex. There are lots of different build system variants, relying on weird poorly-documented helper scripts which have strange interactions. In my opinion, there's just too much magic in the debhelper system to understand how to fix problems. You also have the long, complex and constantly changing list of rules which need to be applied for each update. They are very strict about noting down the copyright of all the files, for example (which I can understand to some extent).
I think the biggest issue is the social problem of the long and arduous process to become a trusted Debian Developer. Although there's now an easier Debian Maintainer option for restricted access, it's still not easy to become one. Other people have to go through a developer to update or add packages. If you can't find a responsive developer, then package changes can sit for months or years in a broken state before they are applied (as I have found out). The whole packaging process is a frustrating exercise in bureaucracy compared to other non-Debian distributions (e.g. Fedora), which is no fun to be involved in for most people.
I think the biggest issue is the social problem of the long and arduous process to become a trusted Debian Developer. Although there's now an easier Debian Maintainer option for restricted access, it's still not easy to become one. Other people have to go through a developer to update or add packages. If you can't find a responsive developer, then package changes can sit for months or years in a broken state before they are applied (as I have found out). The whole packaging process is a frustrating exercise in bureaucracy compared to other non-Debian distributions (e.g. Fedora), which is no fun to be involved in for most people.