You can literally smell it in the air, older cars don't have cats to burn everything uncombusted down to CO2+H2O. You can smell it with a modern car for the first few minutes while cat is heating up. You can see it in car shapes, there's a reason why every modern car looks the same — aerodynamics and pedestrian safety make car shapes converge. You can see it in ubiquitous cans of AdBlue on petrol stations, which was not a thing just two decades ago (and still aren't in many developing countries).

Finally, you can see it numbers: https://www.asm-autos.co.uk/workspace/images/yearly-co2-emis...

There is no fundamental reason why all those changes had to happen, it wasn't the market driving them. It was the regulation.