Goats are wethered for a number of reasons.
Bucklings (young male goats) can breed very young. Young as 3-4 months. You don't want them getting their own mother or sisters pregnant.
Wethers do not stink (unlike bucks which can REEK).
Wethers are easier to handle.
4H & FFA only allow wether goats or does, no bucks.
Most bucklings produced are NOT worthy of being a herd sire. A herd sire should only be the very best of the very best.
Example, it's been three years since I produced one I felt was worthy of becoming a herd sire. I have one this year. I'm very, very picky about who becomes a herd sire. Since your buck is providing 50% of the genetics of your herd, he should be the absolute best possible.
Some people are selling goats as pets. Wethers make excelent pets. Bucks are horrible pets.
Some are being sold to be pack goats. Wethers are excelent pack goats. Bucks are horrible pack goats.
Some people who are purchasing them for meat prefer to eat a goat that has been wethered, and has no "bucky" flavor. (other cultures want only bucks).
If a breeder is wethering their goats before they sell them, they may also be trying to protect their genetic lines. In other words, they do not want someone to pay the price for a butcher quality goat, and then use it as a buck. A breeding quality buck is generally more expensive than a meat quality goat.
Permaculture homesteading/farming over 20 years
Raising meat goats since 1999 (currently 78 goats in my herd)