The only problem is that we can't fully trust the ancient manuscripts we have since none are original. It’s entirely likely that some later scribe manipulated the Savior’s words in the hopes that the Second Coming would be in his day or, at the least, earlier.
If one accepts these prophecies, then it’s clear these prophesies failed, right? Zechariah 12-14 describes the events, not of the Roman assault, but of a future assault during which God would bolster Israel, and the Messiah would rescue the Jews in the day of their redemption. According to prophecy, a number of things had to happen. First, the Jews would be scattered throughout the nations of the earth. Then, wolves in lambs’ clothing would enter the church, not sparing the flock. This would be followed by a “falling away” or mass apostasy. Centuries passed, the keys of the kingdom, or administerial authority, would pass away. The coming of Elijah, who would “restore all things,” was destined to occur, and then the gathering of Judah spoken about in Isaiah 11 would come to pass, followed by the building of the third temple (See Rev. 14). Once established, a great power from the north would come down on the holy city and the Lord would raise up two prophets. Three and a half years later, the Lord comes in his glory and rescues the remnant Jews and destroys Judah’s enemies, leaving only a sixth of the enemy intact.
At that point he descends upon the Mount of Olives, and an earthquake tears a rift into the mount, splitting it asunder. The remnant of the Jews flee into the valley and the Messiah reveals himself, converting the Jewish state in a single day. That’s a lot to happen in one’s lifetime; that’s why I dismiss any prophecies that seem to push the event in the Lord’s generation.