Because they are landed gentry. The Gamgees and other working class hobbits do the field work. Bilbo was wealthy even before his adventure. Pippin and Merry are

Because they are landed gentry. The Gamgees and other working class hobbits do the field work. Bilbo was wealthy even before his adventure. Pippin and Merry are also members of the aristocracy. Farmer Maggot was something more or less like a yeoman farmer. The Shire is, in basically every respect, including its economics, an idealized version of the English countryside.