Freelancers leave platforms because they outgrow the need for them. I wouldn’t always assume they leave one to go to another.
Marketplaces like goLance, Upwork, etc. can have value for new freelancers who don’t have other sources of clients, and are willing to undervalue themselves to compete on price. But once you hit a certain stage in your freelance career you have a portable reputation of your own and other pipelines for new work, and you’re far better off not being tied to any platform, or any extra fees.
That’s just the nature of a freelance business evolving to a professional career.