It would be more general to say - cities are found where people have a reason for wanting to live in large numbers.
Historically that has meant most often natural nexuses of transportation, centers of industrial and other economic activity, and governmental administrative centers. And cities have a natural tendency toward self-reinforcing growth - once economic activity and large numbers of people are located at once place, more of both tends to follow.
In the case of Las Vegas its founding as a major city was due to lax gambling laws, creating a "sin haven", a trick that can pretty much only be done once. Others have copied but with only modest, and often fleeting success - (cough) Atlantic City (cough). Also, it is next to a major river, and grew up at a time that the water rights of the river were not over-subscribed.
Notions of new cities growing up in deserts require an explanation of what it will be attracting people to live there in the first place.
Why is there a new city? All cities require an economic basis, a productive economy. The industries of the future are going to be high skill industries. You will note that new high skill businesses are invariably located in/near existing cities, usually in reasonably nice climates, often where costs are already high, because that's where they have to be to get the workers they need . What is going to attract those educated youngish types out to your new desert city? Even if a super billionaire business type decides he is going to found a new city, let's call it "Bezosville" or "Muskopolis", he may find the competitive *disadvantage* of recruiting employees may be too big a hit to take.