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

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.