How do you know they are future? They were written to first century saints who were told they would shortly come to pass
One thing I look for is Biblical patterns and precedents.
God offered the Jews the Messiah. They rejected Him. They then paid in 70 AD tribulation.
God offers the world a Christ. The world is rejecting Him. Is it not reasonable to believe the world will suffer tribulation for it like the Jews did? How does one figure the world will get a pass when the Jews didn't?
"Shortly" is a relative term. Going back to what you said before, yes every generation has thought theirs was it. And in a way it was. I see that heart attack, that car crash, that 'your-ticket-is-punched moment as your rapture, whether it happens now or 1,000 years ago.
The thing that sets it as now for me is, people have never had the technology before now to have a true world event. 1,000 years ago news from europe took weeks if not months to reach america. Now it happens in milliseconds. That's the stand out feature that makes me think now is more likely than then.
But what do I know.