Because you need people to fill a wide variety of jobs and roles, like teaching, transportation, retail, food service, sanitation, etc. that can’t be done remotely. Unless you are planning for those jobs to be equally lucrative as CEO of Megacorp, you can’t build a functional society without people from various socioeconomic strata. Some neighborhoods can get by with shoving off people in those roles to nearby locations, but commuting time and expense is a limiting factor.