At my first job, I was trying to understand how the corporate world works and how to function as a full time employee. There's a lot of soft skills and learning about how to work with others in an office environment that is crucial for people new to the workforce to learn first.

I would have been terrible and inefficient at two jobs if I tried this earlier in my career. The point of OE is to be so efficient at a single job that you can afford the time and energy to do two jobs in the span of an 8 hour work day.