Because those systems are exorbitantly expensive and require digging up the road to install sensors. If there's a stop sign instead of lights, you need to dig up more private land to run power and set the utility poles to hang the lights from.

A stop sign costs like a hundred bucks, you stick it in the ground, job done. Installing an automated traffic system takes multiple days, a full crew, and heavy equipment.

Plus I'm sure that in today's capitalist hellscape it's also a subscription service that your tax money needs to pay monthly, likely for every individual intersection. Stop signs need maintaining every decade or two.

The answer is money and who's willing to part with it.

On most streets wouldn't you power new traffic lights using the existing power lines that are powering the street lights?