The first reason to believe the predictions of Matthew 24:29-31 are fulfilled is that's the most straightforward way of reading the text. Matthew 24:34 (shortly after) says
"Truly I tell you, this generation will not pass away until all these
things have happened."
To this you can add that it's the most straightforward way of reading Matthew 16:...
I have seen some herculean efforts to foist a first century interpretation on Matt 24:29-31. Most are based on what is unknown and unknowable rather than on historical fact.
We should interpret Matt 24 & 25 (it is a single sermon) as it says it should be interpreted according to the opening verses, viz, Matt 24:2, 3
“Do you see all these things?” He ...
These are the 2 reasons I thought of?! 1) the High Priest knows the court case, the next one may not! It’s his job to pray/intercede for people to make atonement! 2) having his sentence, banishment based on the length of life of another human, a spiritual advocate, places the length of his sentence in Gods hands, not fallible humans! 🤔