Being a traditional employee can guarantee a basic salary every month as long as you are still working, which is a better comparison than being a freelancer who does not have fixed working hours. At times freelance work can earn more money than contract work, but this type of work does not guarantee massive income. I prefer to combine both or with the term "kill two birds with one stone" or more specifically doing one job while earning another income. Doing a side hustle can provide various benefits such as supporting income, developing new skills, financial security and opening up new career opportunities in the future.
One can only do both of these things if one's full-time job doesn't take most of one's time. In some regions, 9 to 5 jobs are pretty good because there is discipline, you get to reach your workplace on time and quickly because of how disciplined the general atmosphere is, little traffic, few disturbances, easy transportation, etc. However, in some regions, a 9 to 5 job doesn't exactly mean you can reach back home at 5:30 or something, but it is going to become night by the time you reach home, and it's because how hectic those regions are.
So, I believe it depends on where a person lives whether they can do both things together because a lot of things can differ based on your region. Also, a freelance job can require a good amount of time because you won't be getting too many clients if you skip their tasks and don't do the work properly due to a shortage of time.