Some regulation achieves this kind of improvement, and we’re probably under regulated in those areas. Particulate matter, for example, is extremely harmful. But many regulations do not have such clear cut costs and benefits.
For example smoke and soot from combustion or dust particles from tires and brakes.
https://www.health.ny.gov/environmental/indoors/air/pmq_a.ht...
Breathing in unhealthy levels of PM2.5 can increase the risk of health problems like heart disease, asthma, and low birth weight. Unhealthy levels can also reduce visibility and cause the air to appear hazy.
Outdoor sources include vehicle exhaust, burning wood, gas and other fuels, and fires. Particle pollution can also travel long distances from its source; for example from wildfires hundreds of miles away. Outdoor particle pollution levels are more likely to be higher on days with little or no wind or air mixing.
It wasn't easily replaced. For many decades there weren't any alternatives for anti-knock additives.
Presumably, you mean there weren't any alternatives for anti-knock additives for around the same price as tetraethyllead.
Octane ratings can be increased sans Pb if needed. Trouble is the extra refining and reduced yield increases costs which consumers weren't prepared to pay for.