Adding more lanes does not “magically create more cars on the road”, but it does mundanely create more traffic, so that increasing traffic lanes provides diminishing returns of reduced congestion.
You have to factor in how many cars are acquired every year, how many people are driving, how they are driving where, and when.
Every year people are buying new cars and the old cars don’t just disappear, more people move to where more people already live, and adding new lanes only invites more drivers to where everybody is already going.
A simple, related and more accessible example is adding parking spaces into a downtown area. This does not lessen congestion but increases congestion as more people drive downtown and everyone drives around looking for a parking space rather than walking an extra 7 minutes from a less congested area.
A similar thing happens with highways, and research backs it up.