Actually some people do, and a lot of people use their existing surveillance cameras for people-counting. see for example https://www.axis.com/en-us/products/ax

Actually some people do, and a lot of people use their existing surveillance cameras for people-counting. see for example https://www.axis.com/en-us/products/axis-people-counter https://www.axis.com/en-us/products/axis-people-counter