> Why is LiDAR still expensive enough for the cost to be a problem for anyone?
Because economy of scale is not the only reason things are expensive. Lidar with 2cm distance resolution has to detect light and measure time to less than 60 picoseconds. It requires sophisticated, expensive sensors like avalanche diodes and complex and extremely fast circuitry.
Consider what a single pixel of a lidar device is doing. It's operating 8 orders of magnitude faster than a normal camera pixel. It's detecting an incredibly small amount of light; the spot of a weak laser on a questionable surface from tens of meters away. Its doing that in the presence of light that is tens or hundreds of thousands of times more powerful than the reflection it's actually looking for. Much of that reflection is even at the same wavelength of the laser!
Even the mechanics of the device are critical. Pixels are gathered sequentially, unlike in a camera, so any vibration makes the data uselessly fuzzy. Vibration in a camera is correlated extremely tightly between millions of pixels. In lidar its correlated with dozens to hundreds. Any high frequency vibration has large negative impacts on the data.
> Or maybe similar tech at another frequency like spread spectrum microwave with phased array semiconductor antennae?
...that's just radar. Literally. Microwaves are 300 Mhz to 300 GHz. Automotive radar is like, 80 GHz, +/=5 GHz (mostly). It uses phased arrays. It uses highly integrated devices. They don't use semiconductor antennas, because the area on a silicon die is far, far to valuable to use for an antenna, but the antennas are incredibly cheap. Radar and lidar are just pretty different.