Because beginners have no idea about OS architecture concepts. If they are a true beginner coming from Windows or MacOS they may not understand things like the Linux boot process. Of course they can read the Arch install procedure which I’ve heard is excellent, but many people are easily intimidated by documentation and often view computers as a tool that should just work out of the box without them needing to understand it. Mint is an attempt at making that happen. Obviously, once you start to modify your Mint install alot you are going to run into issues, and a highly modified or customized system is where distros like Arch and Tumbleweed actually become easier to maintain. I’d argue Mint is a natural first step to the Linux pipeline. People who only need a web browser will probably stop there, while others will continue to explore distros that better fit their needs.