Every apartment and house I've bought up to this day in my life (and I've moved quite a lot over the decades) would have been almost an impossible sale by the owners themselves. Their realtor was able to present the place, reply with empathy (that is, understood where I was coming from and what my interests are) and get back to me with important info about the property and district that the owners themselves usually had little clue about. The owners themselves where often awkward, hard to talk to and overall pretty clueless.

So, all those folks definitely got something out of hiring a realtor. Nothing to do with "legal stuff". Most people are just terrible at presenting and selling things. And that's fine. Claiming otherwise is closing your eyes for what's out there in the real world.