According to the professor, why is a fresco painted before the plaster dries?

Listen to part of a lecture in an art history class.
Professor: As we discussed in our last class, one of the high points in the history of art is certainly the Italian Renaissance of the 14th to the 16th centuries, and surely one main characteristics of the Italian Renaissance was the ceiling of the Sistine Chapel in Rome. So, how did this particular work of art come to be? Well, in 1508 a famous young sculptor was commissioned to paint a new ceiling for this important chapel. His name was Michelangelo. He was commissioned to paint a ceiling with scenes of various well-known stories. This ceiling was huge: over 900 square meters, plus it was 20 meters or more off the ground. Now, the actual term for the kind of painting he did there is "fresco." "Fresco" in Italian means "fresh" or "wet." It means painting directly onto wet plaster. Now, the method wasn’t new, but it became very popular during the Italian Renaissance.
    Anyway, a fresco is a simple enough concept, but much easier said than done. You would have your wall, or in this case a ceiling, already built, and to do a fresco you would take fresh, wet plaster and smooth it onto the wall or ceiling to make a fresh, moist layer, and then paint directly onto that wet plaster. The good thing about this is, as soon as you put on the paint it starts sinking into the plaster, so when the plaster dries, the paint, the color, is locked into it. It has literally become part of the wall or ceiling or whatever and so. lf you do it right, the colors, the images in a fresco, could last a very long time, but remember we said that frescoes are difficult to actually carry out? Well, one big problem is the time limitation. You can only paint while the plaster is wet. As soon as the plaster dries, the paint no longer sinks into it. The paint color would no longer actually become part of the wall or ceiling, so as soon as you lay on a plaster layer, you have to get to work on it with only about 24 hours at most from the time the plaster’s applied until the time it dries, and there’s hardly any room for error either. As soon as the paint touches the plaster, it starts to become part of it. There’s very little opportunity to correct any mistakes.
    Ok, so you can only lay out as much plaster as you can to finish painting that day, but how much is that? Well, typically an artist could do about, well, a rectangle, maybe a meter by a meter and a half, so fresco painting was in this respect radically different from other kinds of painting, from painting on canvases and wooden panels. You were always working against the clock. In fact, there’s a story of one Italian painter who would paint with one brush in each hand just to be more efficient.
    Now, it wasn’t just the art that Michelangelo had to worry about. He also had to worry about logistical details. Aside from getting paints which were really expensive at the time, there was the particularly difficult problem of painting a ceiling, a ceiling of a church that was still being used for ceremonies the entire time it was being painted. And remember, besides being nearly 1,000 square meters in area, the ceiling he had to paint was 20 meters up or more. Is anyone else here scared of heights? To add to the difficulties, this ceiling wasn’t flat, but curved, so Michelangelo needed special scaffolding to do his work. Traditional scaffolding involves building ladders up from the floor with a platform at the top where you can stand or sit or lie down, as Michelangelo may have done, to do your work. This would have been fine for the height and shape and size of the building itself, but all those ladders holding up platforms would have really cluttered the floor, so instead Michelangelo built a system of wooden footbridges that came out from the walls of the chapel.
    So, Michelangelo’s ceiling there in the Sistine Chapel wasn’t just a beautiful work of Renaissance art, but one of the great examples of a very difficult art form, a fresco, with all the logistics involved in this case and the massive kind of undertaking that it was. So, even aside from its beauty and its historical significance, the Sistine Chapel, especially that ceiling, is recognized today as one of the achievements of the Italian Renaissance.