It makes assembly faster and cartridge bearings are relatively inexpensive to produce, there is no fine adjustment required either which is something that requires a human being at the production end or at the bike shop.

Using new bearings from the same lot ensures that they will be consistent in their grading... the numbers you see on bearings like grade 100 mean that the maximum variance is 0.0001 inches and these are considered semi precision while lower grade numbers indicates higher precision and less variance.

For cycling purposes a grade 100 ball bearing is fine although higher grade bearings tend to be better made with higher tolerances, proprietary finishing (often to reduce corrosion), and better materials which last longer. When I worked in the oilfield (manufacturing) we commonly used grade 40 bearings in custom assemblies and on some occasions we also used ceramic bearings where high temperatures and corrosion were an issue.

Ceramic bearings are pretty much useless on a bicycle when you factor in their cost... at about .75 each for grade 5 bearings doing up a bottom bracket would cost $16.50 and the races have to be as precise as the bearings to really make things work their best.

Grade 25 steel bearings cost about a nickel each.