You usually make more per hour working overtime than you would working at a second job. 1.5x is a typical multiplier, which is not a small raise! It's also worth pointing out that not every hourly job is equally mind-numbing.

You cut out all of the overhead: no new processes, policies, and personalities to learn; the work environment is already setup, as is billing; and there’s no chance of conflict of interest concerns.

There’s also no expectation of it continuing, which is good if you don’t want to have to deal with a side project running longer than expected. That’s bad if you’re looking for a true second job but I’ve known plenty of, say, young non-Christians who happily worked on Christmas at a healthy bonus.

Overtime is a far better deal. You don't have to commute or switch environments, just continue doing what you're doing (which may not be all that bad) and make a multiplier based on far you go overtime.

Add in holidays and other shifts and some people can double their income doing overtime which would be impossible to pull off with a second job.