There are safeties, and then there are safeties. Passive safeties are all those mechanical things that keep the gun from firing unless in the proper firing grip and the trigger is pulled.
Drop safeties, firing pin blocks, grip safeties are in this category.
Active safeties are the levers buttons, switches, that you have to put on, and take off.
SOME DA/SA semis have a decocker only. Sigs one like this. Using the lever drops the hammer. The gun is ready to go with a DA trigger pull.
Many others have a combination safety & decocker. Walther, Mauser, some S&Ws, and many others do this. Putting the safety on drops the hammer, but leaves the safety "ON", so that a DA trigger pull will NOT fire the gun. TO make the gun as ready as possible, you put the safety ON, (dropping the hammer), THEN take the safety OFF, so it will fire with a DA pull.
There have been a few revolvers with safeties, some of them were the lever kind, some were not. One I recall was the "Swiss safe" system used on Hammerli SA clones. They used the special cylinder pin to hold the hammer off the full down position, to allow for safe carry of 6 rounds. Same safe condition as achieved by the Ruger transfer bar system, but more cumbersome and had to be manually activated and deactivated.
Another one was the short lived High Standard Crusader. Interesting large frame DA revolver, one of its features was a lever that was both the safety and the cylinder latch, depending on the direction it was moved.
Why no safety on most revolvers? Because its more risk than its worth.
SA revolvers must be manually cocked for each shot. hammer down, properly loaded for the design, they are about as safe as safe gets. Period.
DA revolvers with the hammer down are very safe, these days, we have learned from experience and improved things over previous eras designs. The long, heavy DA pull serves adequately to prevent accidental firing, so a safety lever only really adds the risk of it being ON at the worst possible moment.
Its also a matter of design philosophy, and what the market will buy. Browning thought a grip safety alone was enough on a semi auto pistol. Some of his designs reflect this. He was also big on giving good customers what they wanted, despite his personal ideas, which is why the 1911 had both a grip safety and a thumb safety.
IF asked, why not a safety on a DA revolver, most of us would answer (in our heads, at least) because it would be barking stupid!, if we were honest.
Indeed, no revolver I know of with a safety lever has lasted long on the consumer market. The grip safety ones like the S&W "lemon squeezer" are another matter. There is an actual degree of utility to those.
And, don't get me started on the French, and their ideas of firearms design!
If they could figure out a way to get a magazine disconnect safety on a revolver, I'm sure they would demand it!