Matthew 27:51-54

And behold, the curtain of the temple was torn in two, from top to bottom. And the earth shook, and the rocks were split. The tombs also were opened. And many bodies of the saints who had fallen asleep were raised, and coming out of the tombs after his resurrection they went into the holy city and appeared to many. When the centurion and those who were with him, keeping watch over Jesus, saw the earthquake and what took place, they were filled with awe and said, “Truly this was the Son of God!”

I struggled with the tomb openings as well. An interpretation I've heard for it is that they were spiritually resurrected, to show that they were free from Hades and appeared in spirit (Christians commonly refer to this as "The Harrowing of Hell") to show that. There isn't much of an indication that they were there for too long - the tombs breaking open could have been a result of the earthquake as well.

I think it is important to remember how records survived- There is no historical written record of Pompeii (which likely held a lot of high ranking Romans) being destroyed. Just a single reference to it by Pliny the Younger. It was likely witnessed by a quarter of a million people, though, yet all we know about it is archaeology. So I believe it is actually completely possible that the only written record we have of the saints breaking free from the tombs in a rebellious city on the edge of the empire is from Matthew. John even said more stuff happened that he couldn't even write down.

John 21:25 ESV

Now there are also many other things that Jesus did. Were every one of them to be written, I suppose that the world itself could not contain the books that would be written.

It is also worth noting a similar objection existed to the existence of Pontius Pilate until 1961 when they found a rock with his name carved onto it, and from there it was treated as historical fact.

I believe how stuff was recorded then compared to now differs greatly. Something happens in a village here and several articles are written and published for the world to see. While back then, someone had to write it down on paper, and for that to survive until now the paper had to either not get destroyed over 200 years, or be copied several times.