Local affiliates are independent stations with their own production departments. Some shows get picked up for national syndication (e.g. Car Talk from WBUR, Wait Wait Don't Tell Me WHYY, On The Media WNYC, Prarie Home Companion by Minnesota Public Radio ) while other shows might target the local market which won't make sense as a national broadcast.
Local affiliates can cross license content directly between each other, or from NPR, PRI, BBC, American Public Radio, and Public Radio Exchange, etc.
Local affiliates are not simple repeater stations, as you seem to believe.