Whenever people come up with low numbers like that their mental model is really what they see as sticking some ASCII on the screen with a couple of colors blitted in. Doing text rendering that is actually good, transparency, shadows, the things that make a UI worth using makes things somewhat more expensive. It doesn't have to be overbearingly expensive–modern computers are fast, after all–but it's definitely going to be more than what you think an imgui thing is going to take. And this is pretty important when you're making an OS, otherwise you're really making something that a lot of people will not want to or be able to use.