Most of my rc car engines have metal caged bearings, now that I think about it. Those engines turn 33,000rpm+, and the bearings never explode. What's so different with airplane engines?
If you don't expect the engine to run 30,000K then there is no need to go to the expense of putting bearings rated for that. So they use bearings good for 15,000 to 20,000K. So if you plan to put on a small prop and rev it to that then you will need the harder and higher tolerance bearings that will do that.