"It's so easy when you don't know how". I'm not sure if this phrase is in common use at all, or if I just misheard it once and attributed it to mean that when the details of a problem aren't obvious, its easy to conclude the solution is simple. "Why don't they just do ___?"

At the companies I've worked at, I refer to this as the "well, can't you just...?"

Yeah, I can "just" after I "just" do A, and B, and C, and D, and E, and F, and G.

Drives me batty on top of being insulting. "Surely you realize I thought about that weeks ago, and if it were that simple, we wouldn't be having this conversation."

But hey, I get paid every 2 weeks.