With the right training, many of these trades are not that hard. 
Lot of people think blue collar means back breaking work, that isnt always the case. 

BlokchainB

Construction_and_Engineering

6 Ways the Construction Industry Can Retain Gen Z Workers

The problem is that blue collar work is still looked at as undesirable. 
The older generation is leaving the trade, but they arent passing on their skills to the younger generation. 
Especially with boiler operators, many are from the older generation and are leaving a big gap in this area. 