Because they're not? They both have the exact same history pre-incursion and until Nimoy/Spock's death, even featured a character that linked them both even more strongly. How is that not "inextricably and forever linked," particularly in regards to the shared history?They have a common history pre-incursion, but how are they "inextricably and forever linked" when they are now totally and forever separate?
That's like saying that if we start a Mars colony, anyone who goes to Mars is now no longer linked to Earth in any way, shape, or form, and never has been and never will be. It's kind of silly.