Carbon dioxide is generally heavier than oxygen and nitrogen. Why do it not form lower layer in the atmosphere?

At room temperatures and higher temperatures, the gases have a lot of mobility. The molecules have high velocity and due to winds too, they mix and flow in all directions. So all the Carbon dioxide does not settle at the lowest altitude. Also, the amount of CO2 keeps changing due to the generation and depletion of CO2 by living beings and plants respectively. This cycle continues.

In short, due to the intermixing of gases, CO2 does not settle down forming a separate lower layer and is found in all layers of atmosphere.

But if some people remain for a long duration in a totally sealed room, then the CO2 concentration increases and settles at the lower altitudes and that is dangerous to life.