We had guys wash out of the apprenticeship all the time. It's hard dirty work, and contrary to what slayer seems to think, not everyone can do it. Sure, you can probably learn the basics...how to start the equipment, what the various controls do, but making that piece of equipment perform correctly isn't easy...and many people just don't have the aptitude for it.
Same with any of the skilled trades. I can cut and thread pipe...but I'll never be a plumber, I understand the basics of pipefitting, how to measure a run, how to fit the various connections, etc., but I'll never be a fitter.
I can butcher wood, drive nails, etc, but I'll never be a carpenter.
Of all the "other skilled trades," the only one that I could actually do well is ironworking...but not rod busting. Too much bending over...all fucking day, bent over tying rods.
My apprenticeship taught me to weld, (had to become certified on mild steel) mechanics, (diesel and gas, hydraulics, and some electrical, and proper maintenance.
Not everyone wants to spend 10-12 hours crawling around a piece of dirty, hot, greasy equipment with a grease gun in hand, changing oils and filters, repairing things that you find while you're doing maintenance.
I made decent money in the trades. My last job was about $35/hr, and all the overtime I wanted...and then some. My last couple of years, I made over $90K.
It was easy to make $80K/yr...if I hadn't been commuting so far every day, (~100 miles each way) I could have easily made over $100k.
Do ALL heavy equipment operators around here make that? No...many don't want the overtime, don't want to work in the bad weather, (or can't...dirt work shuts down in the rain) and lots of operators only want to work enough hours in a year to get their pension credit and pay for the medical insurance for the year.
The skilled trades aren't for everyone...and not everyone can do them.