"Sufficiently round" is harder than it may seem. A roller or needle bearing would be immensely easier (cylindrical grinding between centres is a bit of a doddle

"Sufficiently round" is harder than it may seem. A roller or needle bearing would be immensely easier (cylindrical grinding between centres is a bit of a doddle compared to spherical accuracy)... and then you've still got the problem of making them sufficiently precisely that all of the rollers actually bear. Even today you make 'em in quantity first and sort them afterwards. A well-lapped plain bearing/bushing would work better than a bad roller. (Yes, you can make something that looks like a ball or roller bearing even on a cheap 3D printer, and it may even have less friction than the thing that looks like a plain bearing/bushing you can make on the same machine, but it's not going to roll freely and take a load.)