Millionaires that (should) have free time and don't have to work still hire other people to maintain their lawns. Even middle class people pay other people to do that.
Most likely because it's still a lot of work and involves getting dirty, even if the end result is a beautiful landscape.
So imagine working 9 hours a day getting dirty, scratches and whatever, just to beautify someone else's landscape and not getting compensated.
I just don't see human doing that, unless that groundskeeper is getting paid in Federation Credits, that he can use if he leaves the planet.
But he didn't look like the type to leave earth an go spending money buying things from other cultures.
My theory is that humans possibly don't use money, as long as they don't leave earth. If they do, they're financially helpless.
But still it looks very strange to see humans doing stressful, work-intense jobs for absolutely nothing--either something else is going on, or the whole thing is weird.