From the agent's perspective, it's good to drive people to their company's website rather than one of the big aggregate ones. Having more people visit it means they're more likely to do business with that company specifically. Getting people to visit your company's site is pretty straightforward: they put up ads there earlier, often so much earlier that by the time you see an ad on an aggregate site, it's already fully booked for viewings.
And that's how we end up with dozens of different company sites, each crappy in their own way, and in the middle of it all, people trying to find a home pulling their hair out, refreshing a bunch of different websites every few hours.