Since there is no legal reason why the wait, the only people who can answer the question as to why the wait is the heirs themselves. Since you don't want to ask them, there is no possible answer to your question.
Why do you think that an easement is required?
Your Texas Supreme Court ruled to clarify the doctrines in 2014.
I will assume and bet on the fact that there was unity of title for your property and neighbor's property somewhere in the chain of title. If that is not correct, please give us some history.
I'm not going to restate what the case law is or what the elements of an easement by necessity are because you can read the case linked to above.
Your neighbor can and may sue you in a quiet title suit if you impede their use of the access they have to their property. And INMO would likely win.
I suggest that educate yourself and speak to an attorney that actually practices land use law. You're not as smart as you think you are when it comes to easement law.
What exactly does the neighbor need to purchase for a million dollars? They already own the land. Are you saying that for you to grant a recorded access easement it would cost them a million dollars? If that is the case, you're delusional.