debian is substantially older than all of those distros, and you named three that happen to, in my view, been designed specifically in reaction against the debian style of maintenance (creating an harmonious, stable set of packages that's less vulnerable to upstream changes), so it's strange to say that debian is the odd one out
keeping a debian-hosted copy of all source used to build packages seems like a very reasonable defensive move to minimize external infrastructure dependencies and is key in the reproducible builds process
there's definitely a conflict with the modern style of volatile language-specific package management, and I don't think debian's approach is ideal, but there's a reason so many people use debian as a base system
also it seems like the idea of maintaining stable branches of software has fallen out of vogue in general
keeping a debian-hosted copy of all source used to build packages seems like a very reasonable defensive move to minimize external infrastructure dependencies and is key in the reproducible builds process
there's definitely a conflict with the modern style of volatile language-specific package management, and I don't think debian's approach is ideal, but there's a reason so many people use debian as a base system
also it seems like the idea of maintaining stable branches of software has fallen out of vogue in general