That would require effort.
You can say that about anything though. I get paid to provide IT support to people. Why aren't they putting in the effort to learn how to fix their own PC? It would save them money. Why does my job even exist when people can just put some effort in? Why does any job, when we could all be self-sufficient with more effort?
At some point, people value their time more than the amount of money they have to pay to get a job done, whether that's because of the knowledge, time, contacts or even just the sheer graft required to get it done effectively, efficiently or even at all. People's perceptions of where the balance between the two will vary based on multiple factors.
Personally I don't have a view on estate agents as I've only ever bought this house, not had to try and sell through one.
Edit - I will say I feel the same way as some of you about travel agents. Blows my mind that some people (especially 50+) still go and speak to someone to sort out their holidays. Admittedly I've only ever travelled in the internet era but it's not exactly difficult to book a flight and find a hotel yourself these days.