moistfinger: The first reason is: there seemingly is still the bug causing to repeat the same dice roll with too high probability, when you go from one Backgammon game directly to another (with "submit and go to next game of this type").

Second reason: many players have the impression that the dice "just feel wrong" on this site. Me too. (That can be mistaken, of course; it is difficult to tell without statistical analysis.)

Third reason: there have been other bugs in the past, concerning dice rolling. At least one was proven by statistics: the first and second roll in a game have too often been equal. This bug is removed, however, but it shows that the implementation of the dice was erroneous from the beginning, and it still is not ok yet (see first point above). That thing seems overly complicated or inherently bad and therefore difficult to fix.

Fourth and most important reason: the random number generation is not documented, so that the players don't know what they are using. It is as if someone brought a bag full of self-made dice to the Backgammon world championship and claims these are better than any other existing dice (or at least not worse), and the tournament director decides to use them without testing. I think you agree that this would be a bad idea.
It is possible (and not very difficult) to create a cryptographically secure random number generator to roll the dice, that uses for every game a secret key which can be published after the game is over, and the generator code can also be public, so the players know the mechanism and can test it for it's general quality and as well for it's correct implementation, because once the key is known, the dice rolls can be re-calculated with own implementation of the generator. The rolls are however not predictable as long as the key is secret.
Such a method would IMO be required for serious online play of games with random.