They're attractive and get tons of sponsorship and funding from idiots who live vicariously through them.
As far as sailing around the world, it takes surprisingly little (check out the Badger, a small dory sailboat whose owners famously lived on like 4k GBP per year or something sailing all over lol). But to do it comfortably and luxuriously considerably more since you'll want a rather larger boat and they're not cheap to buy or to maintain. But still, actually being out at sea your costs are surprisingly low, because what do you need making a month long passage? You'll have water stores and a water maker, probably eating cheap canned/dry food and fish you catch. Other than ongoing expenses like a boat mortgage or fuel, sat phone, and so on you just kinda chill and do whatever.
Also you can check out the S/V Delos website IIRC they have a pretty thorough cost break down, though nowadays they're entirely fundedby youtube ads, patrons, etc.