Populations in the absence of active constraints (e.g. lack of food) or consumption (e.g. predators or something that rhymes with shmashmortion) grows at a rate proportional to the current population. Anyone who has ever seen a differential equation can tell you that the overall population is then an exponential function of time. In humans, the doubling time constant is about 40 years - not coincidentally, very similar to the length of a woman's fertility. Thus, since we are not lacking food in the US and we are generally not being eaten by wolves, there would naturally always be twice as many people in each subsequent generation. Look at the US census data and you'll see that this is true with a few notable exceptions: WW I, WW II, and now post-Roe v Wade. A quick plot of the census data pre-1910 (before WW I) yields an exponential coefficient of 0.026, while from 1960 onward, it is 0.010. These correspond to time constants of 38 years and 100 years, respectively. Thus, the number of people paying in to SS per recipient is decreasing at a dramatic rate as the population ages, fewer people enter the workforce, and more people retire. It's just math.
