If I bought some land, built a windmill with my bare hands out of stone and wood that I cut and shopped myself, and became the flour producer for the entire region, people tend to say it's a deserved reward.

If I bought that company, paid people to do all the work setting up a dozen more windmills, paid people to manage the new bigger company, and then sold it again to the highest bidder, now suddenly I'm rent-seeking and profiting off of the poor. Even if those things wouldn't have gotten done without this intervention.

It's like a Ship of Theseus: If you split up the flour company into its constituent pieces and slowly hire other people to do them, at what point does it become unethical rent-seeking?

Even someone who buys an income stream solely to ride it out with the minimum amount of maintenance is at least taking a risk to their capital. And even if they protect it with an insurance policy or similar, they're at least locking up the money for a dedicated purpose usually for an extended period of time. People are often afraid of taking risk or limiting their options, so people who do so are providing some value.