>>27280009You can only not overwrite them because in Gen 1 you had a single inventory for everything(gen 2 onwards items began being divided by category, including one for TMs/HMs), and since inventory had a limitation for how many items it could carry, you could deposit HMs to make space, potentially softlocking yourself if you overwrote a HM with another move and had depositted it before going somewhere that needed said HM to get out of.
There's no reason not to simply make HMs non-depositable and letting them being overwritten by normal moves
