Real estate isn't a great example because the carrying costs of unsold property in the form of upkeep, property taxes, mortgage, insurance, etc., are non-trivial. In general it's not remotely reasonable to hold onto property for 20 years without selling it. By contrast, the holding cost on a domain name is effectively zero, so there's much less pressure to get rid of one quickly. Plus, seeing as how the prices have only continued climbing over time, people who held out earlier and then got a good deal more recently don't regret it.
If I had a good portfolio of potential $1M+ domain names I too would hold onto them and wait for the right buyer to come along. I'm in no desperate need of money right now.