First off, NEVER sell your home without a real estate agent. You could be overpricing/underpricing it and really hurt yourself. Example: You sell your home "for sale by owner" and get $250,000 for it. Congratulations, you just saved $15,000 by avoiding the real estate agent's commission. However, a real estate agent has better marketing resources and knows values better than you do. They could have sold it for $300,000. Oops, you could have gotten $50,000 more for your house.
Another reason to use an agent is legality - there are so many little tricks that sneaky people can employ to sue you after you sell your house. Real estate agents have safeguards and checks in place to prevent this and are constantly updating their paperwork/forms.
But to answer your question: it takes a lot of time and effort on the part of *both* the buying agent and the selling agent to complete the transaction of a home-purchase. It's not something that just anyone can do. In the case of an agent-assisted buyer in the purchase of a "by-owner" seller, an agent basically has to do the work of BOTH the buyer's agent AND the seller's agent, with a questionable amount of commission. This is why "by-owner" sales usually short-change the seller - no real estate agent is going to show/recommend the house to their client... so as a seller, you are missing out on a huge portion of your target market and will have to settle.