> I've even designed an analog hall effect keyboard PCB with integrated IR sensor, dual power regulators (to handle 95 ultra bright RGB LEDs), invented-by-me analog hall effect rotary encoders (incremental and absolute), and more. It wasn't rocket science.
Sorry to burst your bubble...
Glad you learned enough to do it and had fun with it.
Yet, such PCB's are trivial to design. Heck, one could auto-route something like that and get a working board for prototyping. In fact, I have done exactly that many times over the last four decades for keyboard/control-panel boards. And auto-routers suck. The fact that one can actually use one for a PCB is a good indicator of how trivial that design might be.
One of the big differences between hobby PCB's and professional EE-driven PCB's is in manufacturing and reliability.
It's one thing to make one or a few of something, anything. Quite another to make hundreds, thousands, tens of thousands, millions. As an example, I am pretty sure you did not run your design through safety, environmental, vibration, susceptibility and emissions testing.
For an example of complex design one can look at such things as almost any dynamic RAM implementation, from SDR to DDRn. Timing, signal integrity and power integrity are a big deal and can make a massive difference in performance and reliability.
Another example is just-about any PCB used in automotive designs. They have to survive brutal power, thermal, vibration and RF environments for decades. This is not trivial.
Other fields with critical needs are medical, aerospace (which includes civilian flight) and industrial.
Consumer electronics is actually quite critical at the limit because you are dealing with very large numbers of units being manufactured. In other words, while a design for something like an industrial CNC machine might only require a few hundred or a few thousands of boards per year, in consumer electronics one can easily be in a situation where we are running 50K to 200K boards per month. Bad designs can literally sink a company.
I understand though. From the frame of reference of a hobbyist or enthusiast everything can look simple. That's pretty much because they just don't have enough knowledge or information. This means they only have access to the most superficial of constraints, which makes PCB's seem easy, maybe even trivial.
As my wife likes to say: A google search is not a substitute for my medical degree.