You can look around the world, janitors consistently earn more in countries with a stronger middle class.
What you are calling "boom periods" are not composed of lots of people upskilling and moving into a higher earning partition of the middle class (or leaving poverty). I have no idea why you expected then to behave like something they are not.
What you are calling "boom periods" are not composed of lots of people upskilling and moving into a higher earning partition of the middle class (or leaving poverty). I have no idea why you expected then to behave like something they are not.