The problem is that by the time a star ship is so badly damaged that there is no possible hope for it and the best move is to sacrifice everyone on board for chance of doing some damage to the cube then the ship is normally too badly damaged to actually go fast enough and survive enough shots to actually ram the cube in the first place.
As for an unmanned shuttle it would need to move extremely fast in an evasive way to reach the cube. The faster you go the more your mistakes matter, imagine travelling at the speed of light and being one degree out. This means that your programming would have to be absolutely perfect even though you would not be able to reliably determine where the cube was going to be in relation to yourself (assuming the craft was launched from your ship). If you get all that right then it only takes the cube to get a good shot in (even if it misses it can knock you off course) or the cube to move slightly and it is all for nothing. Even if you hit it then it still might not do a significant amount of damage unless.
Although, I am not aware of anyone ever saying in universe that they had never done this.