There have been a few adult lessons for years. But if there are three times as many adults as kids in the U.S., you'd think there would be three times as many adult lessons. There aren't. And it's not because adults know how to swim. A Gallup Poll showed that 46% of American adults were afraid in water over their heads in pools. If they're afraid, they understand (know) the water and they're not safe: they cannot swim.
Adult lessons have never been profitable because as an offering, they have never worked well. Large numbers of adults quit lessons for reasons this site makes clear. In addition, many instructors are afraid to teach adults: what if the students panic? What if a student is bigger than the instructor?
It seems that no one ever asked, "Why do adults quit my lessons? Could it be the teaching?" Apparently, no one called the students who quit to get their feedback, or to fix a teaching problem they might have heard about. No one questioned the value of the teaching, lessons, or system. No one asked students what they wanted to learn—or examined the definition of "I can swim." Perhaps there was no need to examine it because office personnel and swim instructors at big agencies didn't need to get the definition right in order to keep the lights on or put food on the table. They just took registrations for the next group of beginners.