the reason that the entire assy. gets replaced is because trying to remove the bearings will, usually, ruin the cups, and pressing in the new bearings will, most likely, not go well, and requires an exact fit bearing installer tool.... the cups are thin wall aluminum... just supporting the cup without damaging the threads is a challenge for most folks..

ok, so you figure you can do the task, and, by a twist of luck, you get the bearings into the cups... guess what? the cup has been stretched and worn by the replace and the bearings are now too loose in those thin cups.... not a good thing.

the consensus, after a few lengthy discussions online, and not just here on BikeFo, is... get the new assembly.

there are replacements available for between $20 and $110 on Ebay... the expensive one is an Omni Titanium beauty with Ceramic bearings.. new, direct from Omni.
most are in the 30 to $50 range.
new bearings alone cost about $20 to $30...