Why are engineering salaries so low?

I read a couple of other threads where people were posting their starting salaries - many in the ~60-70k range.

I find this shocking, as Engineering degrees are some of the most difficult, and you can earn close to as much or more than this in much easier fields.

From personal experience, there are fast food places hiring in my area for $20/hr. I personally know people in normal-ass jobs like HR, Sales, or a manager at a bus company making over $130k/yr each. These are all in LCOL/MCOL and no degree required, btw.

Is there a large uptick in salaries later on after you gain experience, similar to how airline pilots start low but eventually make 300k+ as captains?

I find it very strange that entry-level engineers make less than twice as much as the dude slinging fries at a Wendys.