According to this article, Roman dice did not always have the modern cube-shaped form:
[I]n contrast with modern dice, they weren’t always exactly cubes. Sometimes they were noticeably a bit flatter than a true cube, or a bit longer, making them more likely to fall on some faces than on others.
A possible explanation is provided:
Did it matter to game players that these dice were not fair? “We don’t know for sure,” Eerkens says. The way Romans wrote about dice falls suggests they were regarded as signs of supernatural favor or of a player’s fortune, however. The archaeologist Ellen Swift, in her book Roman Artifacts and Society, writes that high rolls had associations of benevolence and felicity, and that rolling three sixes at the same time seems to have been called a Venus. “Dice potentially played an important role in conceptualizing divine action in the world,” she writes.
If you thought that your prayers had more to do with a roll’s outcome than anything else, perhaps a perfect shape wasn’t required. Eerkens says, “Some of the non-symmetry that we see in the earlier dice might be a by-product [of the idea] that it wasn’t thought to be very important in the function of the dice—that it didn’t matter too much, because other things were controlling whether you would win or lose the game.”
This is ingenious but also a but tenuous, in my opinion.
I have an alternative explanation: as is well known, dice were made of bones and bones naturally tend to have an elongated shape.
Is there some known research on this subject?