Given the fact that industries emit about 1.5 billion metric tons of carbon dioxide each year just in the US, why is it still such a small part of the atmosphere's composition (0.04%)?
The mass of the atmosphere is 5.1 × 1018 kg, which is 5.1 × 1015 t.
As stated in the edited question,
industries emits 1.5 billion metric tons of carbon dioxide each year,
That's 1.5 × 109 t.
The mass of Earth's atmosphere is 3,400,000 times the mass of CO2 that industries produces each year in the US.
As well as the atmosphere having a lot of mass, there are many processes which remove CO2 from the atmosphere, see the wikipedia article for the carbon cycle. Plants sequester it as biomass, some of which ultimately ends up in the soil long-term (the rest is either released again by organic decay or fires). The ocean also absorbs a lot, which is related to the problem of ocean acidification you may have heard of.
It's a delicately balanced dynamic equilibrium that we are disrupting, not a static situation. Think of it like a sink with a half blocked drain that's slowly filling up as you run the water, rather than an empty bowl you're filling up.
"Small" is a vague and slippery term. There is now a lot more CO2 in the atmosphere than there was a century ago and that change has consequences, especially to climate stability. There is a lot more human emissions - around 40 billion metric tons per year globally now - and that is causing atmospheric concentrations to rise. Those emissions are in addition to natural sources and are moderated by natural processes that absorb and recycle Carbon and overall, so far, natural processes have been taking CO2 back out of the atmosphere at rates greater all the natural CO2 flow into the atmosphere, or the human addition would be making concentrations rise faster than they are.
This diagram shows The Carbon Cycle. It is a few years out of date - the amounts of fossil fuel burning and other numbers will have changed a bit but it gives a good overview of where the carbon is coming from and going to.