In some cases, speculators provide actual value. E.g., they prove or maintain the assets that they speculate on. In others, as with financial speculators, they provide liquidity to the market.
Neither is true for domain speculators. They get paid for making the world worse.
Some domainners have lots of domains & develop lots of them into valuable properties. Sure - they have a few that are parked - but that's a percentage.
Typically - money is a measure of value you add to the world. They have to be adding some sort of value in the world or they wouldn't be able to extract value from their activities.
The classic medieval example is a bandit or a warlord who take over a section of road and extracts tolls. He didn't build the road, but he can extract plenty of money as long as he controls it.