Explain Like I'm Five

When the resistance goes high, why is that called high impedance, instead of something like high resistance?

Disclaimer that I'm not actually an electrical engineer, but I'm pretty sure it's just convention. Positive charge is also an absence of electrons, just because Ben Franklin guessed wrong, and conventional current goes the opposite direction to the actual current. I probably would have called it "floating", "disconnected", "stopped" or "open", if it was up to me.

As for the manufacturing, making a MOSFET is as easy as taking a wafer of silicon, doping a couple of spots to be opposite to the bulk next to each other, and then oxidising a spot on top of the channel in between to form an insulating SiO2 gate barrier. This is good in terms of steps needed, chemical precision needed, and number of features required per transistor which translates to more transistors per area. CMOS allows an entire chip to be printed in place with barely any more steps, by using both N and P type MOSFETs in complement (Complementary Metal Oxide Semiconductor) to ensure that there's always a path available for current. Then, the only thing left to do is start building up the interconnecting wires over top of the semiconductor with vapour deposition or similar.

There's a video series where Sam Zeloof makes a MOSFET from scratch in his garage. Skip to the second video if you don't care about the theory so much. Wikipedia also has a nice illustration of the process of printing CMOS in more detail.