I have wondered the same. Block walls help increase privacy because our homes are often built close together and next to busy streets, which is the result of high land values. See:
Why do all the houses have block walls around their property? - Las Vegas Sun Newspaper
Land values are surprisingly high in Vegas despite there being a lot of vacant desert land surrounding the city. About 85% of Nevada is owned by the federal government; our government occasionally sells land parcels at auctions. See
BLM to sell 754.78 acres of public land in Las Vegas Valley | BUREAU OF LAND MANAGEMENT. The land is likely mostly purchased by investors or developers. (In the recent 2018 auction, of the 20 parcels won, only three were by non-business entities, see
https://www.blm.gov/sites/blm.gov/files/documents/files/SNPLMA_Winning Bidder report Winter 2018.pdf) The investors/businesses want to maximize their investment, hence those homes that are built are often packed in tightly to increase units sold by acre. Most people I have spoke with in Vegas, if they know their size of their home lot at all, know it by the square feet and not by acreage. My lot is about 6,500 square feet. According the Las Vegas Sun article, walls became standard at some point became standard to block noise from streets, neighbors, and provide privacy. If you drive past a new development, you will see that the walls often go up before the homes do.
I came across this information because prior to purchasing my current home, I looked at the possibility of buying land and building my own home. I could not find affordable land anywhere near where I wanted to live. The above were some of the reasons I found when I was searching for explanations on the dearth of land for sell. So, I bought a home like most people in Vegas probably do - built by a major home builder on a mid-sized parcel of land that probably has 20-50 homes with 3-5 different models. I chose where I live because I wanted to be near open desert because I get tired of the feeling of the walls everywhere; many of the developments feel like walkable islands - nice when you are in the development, but not walkable between them - especially where gated communities are prevalent. I also like to explore the desert. I made sure that I was not adjacent to merely BLM managed land, which is typically the type land that goes up for sale eventually unless it is made into some kind of preservation/conservation area. If you buy next to BLM managed land, it could be another home development in a few years if you are fortunate, a modest retail area or a street if you are not. Fortunately, the area next to my home is also a conservation area (Sloan National Conservation Area) that is also managed by the BLM, but because of its designation is less likely to be sold off soon or at all hopefully. Of course, there is also a major flood zone/drainage next to my home, so nothing is going in right next door, except maybe a flood sometime.