Why marginal benefit is decreasing as more and more pizzas are being produced?
You can think of it as the marginal benefit per person. The first piece of pizza you eat is amazing, it has a very high marginal benefit. But say you eat five whole pizzas in one sitting and then are immediately  offered another slice. That next slice doesn’t seem so appealing, does it? You’re already full and starting to get sick of it. That slice has a low marginal benefit. Certainly lower than that first slice. You can also think of this as a continuous process (i.e the second slice is a little less satisfying than the first, and the third is a bit less satisfying than the second and so on).
So, as the total consumption of pizza increases, the marginal benefit (i.e. the benefit of the next unit) gets smaller.