That’s an assumption.
For instance, let’s say that the information from one end of an entanglement gets across 14billion light years in 7 nanoseconds. And it gets across half the universe in 3 ns. We would perceive that as instantaneous and it would take a long time to prove the time-dependence.
Maybe they can affect each other instantly, or maybe they’re just really, superduper fast. If it were beyond our ability to measure at the time, the 2 scenarios would be identical.
The pot of gold behind entangled particles communicating across vast distances so quickly is, well, the ability to communicate across vast distances so quickly. It’s akin to comparing the USsnailMail to the telephone.