factory items are not helicoiled for two reasons. cost being #1, and #2, factory stuff is assembled in the assumption that it will be a one time assembly. if and when it comes time for repair, it will usually be replaced. helicoils [or other types of inserts] are the ONLY choice for items that see extreme or heavy duty usage, and/or are frequently tore down for inspection or wear repair. this is to insure the quality of threads in the parent material, in this case, aluminum, will always stay as intended. if any damage does occur to the threads, the inserts can be changed out without worrying about the integrity of the original insert threads in the parent [aluminum] material.
this does not by any means, suggest you can't disassemble parts that do not have inserts, and ruin the threads. this happens all the time, and threads stay as manufactured. however, if done a lot, or electrolysis between the differing material happens [steel bolt, aluminum threads], the threads will wear out or tear, and the bolt can break or pull the threads out. anti-seize is your best friend in these [infrequent] situations.