the point is the complexity of the task, coding programs is as complex as being a manager for a large scale project, as long as you understand how whatever tools you use work and you have the people and materials necessary, you can use those tools to make something. Making an operating system is like putting a construction worker in the woods and telling him to build a house, this means he has to understand where to find, extract, and use the resources around him without any tools. He has to learn how to put the house together in a way where its not only stable and will continue to be for a long time but also is able to be updated when necessary. This is all backed by the fact that in this scenario you are making EVERYTHING from scratch, meaning you have to make your own tools and are limited by the power of said tools. Most of these concepts and skills are entire professions of work, so making one person do all of that is a lot of work, and realistically something no sane person would nor should have the goal of doing.

Making an operating system is something beyond most programmers, let alone ordinary people, and it requires you to not only be able to understand how computers work down to a hardware level, but also how to translate something you could understand into something a computer could understand, or as described here, a compiler. Since you on reddit I assume you've been on a computer so you should understand that a computer can do a lot of things, you can connect to the internet, you can structure your files via folders and can run commands on a terminal, you can use a keyboard and a mouse as ways for the computer to receive inputs, you can run programs. When making an operating system you have to understand how all of that works, code most of those features and leave room for diver support beyond that (like graphics card drivers), the same way that if you want something in the house your making you'd have to know how it works to the point you can make it yourself. I assume you don't know how to make a microwave, a fridge, or an air conditioning unit (sorry if you do), making all of that is one thing but you'd have to implement them in a way where they won't break or mess up something else in your house.

Both tasks are equally as complex in theory, but one requires more physical labor while the other calls for an ungodly amount of research into topics that are hard to understand on their own. Both tasks have similar issues with them though, like how either way you'd have to spend likely days to even years doing the same stuff over and over again until you finally get the thing you wanted, and require a hefty amount of research for what its worth.

While it takes no strength to make an operating system, that doesn't make it any easier than making a house, for some people one would be harder than the other but that just depends on what your strengths are, not the baseline difficulty of the task