CO2 levels rise to unhealthy levels (700ppm) within a few hours of a typical bedroom if you keep the doors and windows closed. Its also why Submarines have CO2 scrubbers.
Because the levels in the global rise are negligible in comparison to what is dangerous for breathing. Here's a hint, if you think you've spotted something blindingly obvious in a field scrutinized by hundreds of thousands of scientists, you're probably making a mistake.
Here's a study showing reduced cognitive performance at various CO2 levels in the ranges we're talking about.