At this point in time, why can't we do smart crossings? Like have some sensors and software to change lights based on real-time traffic conditions.

You don't have this? In Sweden we have sensors to detect cars, pedestrians and bicycles to shift the lights as appropriate. During rush-hour those features are turned off/discarded in favor of "grid optimized" timings. In Netherlands they prioritize pedestrians and cyclists when it's raining.

We also have LED lights in our traffic lights which I've come to understand is a saftey hazard in USA because snow falls sometimes.

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.

Assuming you're referring to the US, we do. They're all over the place. But they're a lot more expensive and complicated than roundabouts and depending on the traffic pattern they can still be less efficient.