Statistics. The article says mortality after 100 is about 50%. Which means out of everyone who reaches 100 years, only one in a million will reach 120 years. Since there are only half a million humans in the world who are older than 100 years, people over 120 years should be exceedingly rare.

So even if there is no upper limit to the human lifespan, and mortality doesn't increase after 100, it would be very rare to see people older than 120.