Really? How so? How is dependency resolution managed to the point that it's relatively painless without resorting to automatic dependency management? I'm not saying I don't believe you, I'm just curious. Slackware was the first distribution I ever used oh so many moons ago that I've forgotten how it all worked. Ah sweet nostalgia as someone else mentioned.

Libraries are built as a single package, not split into separate "-dev" packages with headers, so there are fewer packages to manage. Also, these days it's easy to just install the whole of Slackware for a desktop system (when even an SSD is hundreds of gigabytes, a few gigs is no hardship.) So when you do need to build third party applications, your system probably already has what you need. Slackware already comes with a good set of the major libraries, even if they're not GNU (pcre, for example.)

Slackbuilds.org will tell you if one of the packages there has dependencies that aren't in Slackware. Typically installing them is not onerous.