The issue is just a bottleneck at the sawmills, likely due to labor shortage or covid restrictions.

There's still an oversupply of trees, and (unfortunately) tree farmers have not seen any increase in price for the raw lumber.

Similar things happened with the meat processing plants and food/vegetable packaging.

why is Lumber expensive? the only rationale I could think of is that housing construction has spiked due to low interest rates.

Some anecdata from a neighbor who works for Lennar... The Lennar Corporation is still buying up property with plans to develop "in the future" but they are not going to be moving forward with most of their development plans for this year and next. She is actually worried about losing her job in the next 6 months because of the way things are moving in that world.