Because the transactional paperwork can already be filled out, even by real estate lawyers, for a couple grand today. No one is paying 5% for that, even though would-be disrupters talk about it that way. Think about what it would take to transfer your house to a family member, assuming it was paid off. That paperwork would be ~$500 per lawyer involved. Anything more that you're paying for an arms-length sale transaction isn't related to the paperwork processing part of it, IMO.

What's really being paid for is access to MLS, advertising, labor to drive people around, maintaining the brokers office, people on both sides who talk crazy people off the ledge and keep deals together when someone is flipping out over a $1000 repair one way or the other.

Even when you "went online", I bet you found the house via a paid insertion into MLS (most likely) or a real estate advertising site. Why did you go there? Because that's where the houses for sale are. Why do people pay so much to put their house in MLS? Because that's where the buyers go to search to come up with their short-list of houses to go visit. Why should MLS listings be so expensive? Because they've built the marketplace. Same as why Amazon or Ebay can charge so much.