The "true work" is sporadic. A business will need an engineer to work hard for long hours for a few weeks, then they won't need him at all for weeks more except to be on hand if something goes wrong. Then maybe some more work, and even longer lulls.

But if you paid them hourly, they'd starve or fuck off to another job during a lull, and then where would you be when you needed them again 3 or 4 months later? Similarly, salaries don't really work any better either, because there's this psychological expectation that there will be regular duties to perform for that weekly paycheck. Psychological expectations for all parties involved. These systems have evolved and adapted to cater to those psychological needs. They keep the extra engineers on hand, cosplaying, in case there is work for them, so that they could in theory start working immediately (the hiring cycle is brutal, but the learning curve to make them useful is worse).

Even those involved aren't typically aware that this is what's going on, if they became aware of it they'd be forced by convention to try to come up with a new system that was more efficient in one way or another, but that's impossible on practical grounds (disincentivizes key personnel such that businesses which attempt it tend to fail). When this does happen, quite often there are lots of comical stories that come out of it (for instance, believing that because these people tend to do little in the way of constant work that they can be replaced by people who are wholly unqualified, because unqualified people can screw off just as easily as the qualified).