Most desert cities were founded before electricity, let alone air conditioning. Phoenix, Arizona was apparently founded as a farming town, and was quite sustainable at its early population.

You can't stop people from moving and aggregating. Relocating hundreds of millions of people into freshly-built temperate cities is obviously impractical, not to mention more resource-intensive in the short and medium term than air conditioning their current residences. We have to develop techniques that work with the world as it is, not just pine for utopias that might have been.