Because exponentials will eventually break the laws of physics.
For instance there is a relationship between information and energy, so a system processing an exponential amount of information will need an exponential amount of energy, which the universe will eventually run out of.
At this point we know very little and we have to leave it to the AIs to determine if physics has any limits.
That's a tall order. Despite all the fancy news headlines, which I must admit gave me existential anxiety at some point, it seems that AI still can't truly make a discovery. Also I really don't know if we know little or a lot, how can we really tell ? It could be argued we actually know a lot because it is ridiculously hard to make an actual big breakthrough in physics.
Yeah it sure would be convenient if this is the one time in all of nature that a curve that looked like it was going exponential before suddenly conforming to all past trends and becoming sigmoid was actually exponential after all because you really really want it to be.
Why do you think it's an S-shape? As you say, we're heading for a singularity!