It's because Soul Eggs rely on money. So egg 1 could need 100k cash, egg 2 250k, egg 3 500k, egg 4 1m, egg 5 2.5m, egg 6 5m, egg 7 10m, egg 8 25m, egg 9 50m, and egg 10 100m coming full circle. So egg 40 could require 500t with that cycle, but then egg 50 would require 1Q, making it 2000x the cash necessary, but only 1.25x the Soul Eggs.

TLDR: Each Soul Egg needs more money but it grows exponentially, not linearly.