You can do both
You can work a real job while training at a high level. A lot of top runners don't do that just because they don't have to, or they have a crappy degree and wouldn't really make much money to be worth working a real job, or they just don't want to work. But there are plenty of examples of people working jobs related to their degree and continuing to get faster. They just aren't the biggest names because the biggest names are making $100k+ to just run and wouldn't greatly benefit working an entry level job in their field to bring in an extra $60k that would entirely be taxes at their top federal tax rate of 24% or more as well as state income tax. It's just not as much incentive to spend 40 hours per week working when the vast majority of your income comes from not working the real job. For someone with a smaller running contract, like $20k-$40k it is more appealing to start your regular career as well as running.