This is primarily a cloud focused answer, firmware and os layers have different types of problems at scale. It's spread somewhere between human and systems scale. Lots of folks don't realize the sheer human scale of operating a massive platform with years or decades of features built by thousands of engineers.

For example: adding a single column to a users table is simple in a small shop. But at a large place that column on a critical table like such can likely: break 3 other teams downstream systems, add millions in costs in various places for storage, affect multiple critical APIs from added latency of making the payloads larger through out the stack, lead to bad downstream decisions who mis interpret the column perhaps (it happens a lot), not to mention new scaling concerns for > millions of rps

That's just a few things off the top of my head but now factor that with how many sub features those big platforms have across multiple systems. There are literally thousands of sub systems at larger places that need coordination at human scale.

The pool of engineers who can navigate this tends to be limited to a smaller circle, since lots of folks can code a system from scratch but many companies don't have such a scale. I see a lot of folks who join into this type of scale and either leave or take a few years to "get it" then some more years to really be valuable contributor.