Here’s a fun example of why scalability is hard: What problems do you run into by going parallel on your toilet paper dispenser?
Although most homes have only single roll holders, most commercial establishments have long ago switched to devices that hold multiple rolls. We decided to install one, so we purchased holder of two rolls, side by side.
We discovered that although we now had two rolls instead of one, the problem was not solved. Both rolls ran out at the same time. Sure, it took twice as long before the rolls emptied, but we were still stuck with the same problem: no more paper. We had discovered that the switch to two rolls meant we had to use more sophisticated behavior: the algorithm for tearing of paper mattered.
A fun article with a good look into how computer nerds look at the world, but it also hints at actual issues with scalability: More does not automatically equal better.