Programming becomes massively more difficult as you scale - and by scale i mean lines of code, number of programmers, number of programs, complexity of architecture, and more. Humans have 50 years of frameworks, software design methodologies, systems engineering and management practice experience and surprisingly straightforward projects still fail.
Just figuring out what it is you need to build (e.g. software requirements, project backlog, etc.) is so difficult - and excruciatingly painful - that it is its own subject matter with entire texts devoted to it. Are you waterfall? Agile? SAFe agile? Does it even matter*?
Like a lot of things (teaching comes to mind), programming is fun (and sometimes even easy) when you're just programming. But programming within the social, political, fiscal and temporal constraints typically placed upon programmers can make it very difficult very fast.
*It does!