Because the space required to stock such a selection of parts would be ridiculous, not to mention costly with inventory collecting dust.
Off the top of my head, you'd be looking at 5-10 barrel manufacturers(the more well known ones), need to stock different lengths and profiles(or someone would still complain that no shop had what they wanted), a dozen different triggers and trigger groups, a dozen bolt handles, aftermarket bolts, dozens of aftermarket stocks..
And that still doesn't even scratch the surface since you have to consider color selection. Are red triggers popular? Black? Stainless? What happens to inventory you've purchased that isn't selling since it turns out hardly anyone wants a purple extended magazine release?
Unless you want to dedicate yourself to being a 10/22 shop, it just makes more sense to handle it via mailorder, or online these days. You could empty most of the LGS in the bay area, fill them with nothing but 10/22 stuff, and still have customers that can't find what they want.