As a solo, the job is to add value and make the customer happy so you build your reputation. There are definitely solo contractor grifters out there but they usually struggle with the reputation issue.
Agencies develop a grifty dynamic where they realize that their job is to sell themselves and as many headcount and hours as they can squeeze into a contract. Now instead of hiring that solo dev, they’re forcing you to pay 20 hours/week to the project manager and to have two devs minimum on it, one or more of whom are complete juniors who will spend incredible amounts of hours solving simple problems, but that’s okay because that’s what they want.
Agencies rely on their status and size to impress clients, which short circuits the reputation issue and opens the door to these shenanigans. They also target bigger companies where wasting time and moving slowly might be the norm, or at least something they can gloss over by sending the sales people out to schmooze the execs over dinner again.
I knew some excellent devs and infosec people who turned into agency operators who just tried to milk companies for contracts and run a revolving door of junior hires who they can squeeze hours from until they quit. It’s sad to see.
To be fair, I know a few good agencies as well. They tend to be in such high demand from reputation that they’re always booked without even trying. It takes a long, long time to get there though.