I'm offering an in-universe explanation because it immediately made sense to me. It made sense time-travel-wise, and it made sense for the Terminator universe.
The future is not written. No fate but what we make. Sound familiar? This is a theme in the Terminator universe, and it means the Terminator universe is a free-will time travel universe. There are two general theories of time travel: predestination and free will.
In a predestination universe, time is immutable and you cannot change it no matter what you do, even traveling through time. You can travel to the future and meet your future self, but once you learn something of your fate, you cannot change it. For instance, you will know that you returned safely to the past and lived those many long years to meet your younger self. No matter what you do, you are predestined to meet your younger time traveler self. You cannot avoid it and the universe will conspire to ensure that events occur exactly as you remember them (assuming your memory is perfect.)
In a free-will universe, time travel can alter the timeline, or create a new timeline. You cannot learn anything about your own future for certain because you are free to change it. 283
If you were to meet your future self, this would not be consistent because you know your own future but you are also free to change it. To be consistent, the only future where you travel to is one where you disappeared at the moment of your departure. Because you cannot know whether you will return. No fate but what we make.
Remember, there is no older <PERSON> and <PERSON> already living in 2007 who returned to 1997 and grew ten years older. Why? Because their futures haven't been written yet. They haven't returned to 1997 yet and presumably they never will. This agrees with "No fate but what we make."
When young <PERSON> arrives in the future from 2007, there is likewise no older <PERSON>, Resistance Leader already living there. Why would there be? <PERSON> disappeared in 2007. He hasn't returned to 2007 yet, and it remains to be seen whether he will. The future he travels to is necessarily the future where he never returned, because he hasn't returned yet. 674