The self-employed plumber has to accept phone calls when they're in the middle of a job, has to do a whole day's work then spend their evenings lining up future work, has to manage their schedule a long way in advance, has to keep commitments even in the face of illness and family emergencies, has to go around giving quotes to people many of whom are just comparing prices, has to manage cashflow and deliveries, has to make nice even with homeowners who are a pain in the ass, has to be able to sell potential customers on the quality of work they haven't yet done, has to deal with complaints, has to chase payment and deal with nonpaying customers, has to keep accounts and business records properly, and so on.

A jobbing plumber just needs to text his buddies asking if they've got any work for him this week, do the job and leave with cash in hand the same day.

If you're good at plumbing but disorganised or don't have great self-discipline, the latter is a much simpler way of working.

A very significant number of freelancers think of themselves as employees. They struggle with "asking for raises," instead of telling their clients "this is my new rate." They are extremely depressed when a contract isn't renewed, or is terminated early "they fired me. Why?" They are dependent on a single client, let themselves form close relationships with that client's employees, have their working hours set for them, etc.

Going into freelancing, the first thing you should be thinking is "I am a business that employs one person: me."

That is basically franchising, you exchange some cash upfront and ongoing royalties for a brand name and some level of support with training/business planning/marketing/leadgen/software etc. There’s a franchise for just about anything you can think of, and a surprising number local businesses in the US these days are actually franchises, not just fast food restaurants. Some are excellent and some are scams!