Because it's a better experience than the alternatives. On a job board they hand over their cell and email and get contacted by hundreds of agencies. On Messly they stay anonymous until they choose to connect, they only ever hear from a small number of vetted agencies, the outreach they receive is personal and includes real example assignments, and they manage it all from a mobile app.
The length of the process is deliberate. It filters out the window shoppers — the ones who finish it are keen and have some urgency.