Possibly because they know no better. I think most agents will agree that normal applications are simple and most people can do it themselves with little hassle. And save enough money for some furniture when they arrive.
Agents are extremely useful and almost a necessity when applications aren't a straight forward yes to every question.
I would always also check advice given by agents, as when I started my application years ago, I enquired with a migration agency and they informed me that I wasn't eligible for a perm residency visa and would have to go for a 2 year temp visa first. Even when I pointed out that I had enough points they told me that they thought my case wasn't strong enough to apply for a 176 family sponsored visa. I applied myself and it was granted 5 months later. So you can see my hesitation with agents. (I can't mention who the agent is as they are on this site frequently.)