Is is hard for one guy to finish the freelancing job? For example, in software engineering area, some guys can only do backend and some guys can only do frontend, it's hard for them to finish freelancing job individually. So here's the problem, do you want to find co-worker with different skill to help you finish the job?
Whether or not you need a partner or coworker depends on the work you do, the work you take on, and your desired business goals.
It is completely possible to never have a partner and be successful in many fields. It is by no means ever a "requirement" to have a coworker or partner.
If the work you perform has certain expectations which you can not meet yourself, then you may need a reliable subcontractor or co-worker to perform the duties you can't.