There was some recent discussion of this at
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=840832&page=1.I think the short answer from Larian's perspective is that they are giving you the option to freely switch between one melee and one ranged set, which is more than 5e allows, but are still making it cost a new action to swap in a different melee weapon or ranged weapon from your pack or the ground which is why that's handled differently.
There is also the fact that you
should be able to map weapons to your custom slots and then use the number keys (by default) to swap between them, at the cost of an action. However, that functionality is currently very buggy if you want to map more than one weapon and the shortcut keys only seem to work if you have the custom toolbar open. I reported this to Larian support not that long ago and they confirmed these were bugs that should be fixed by full release.
However, if you merely want to swap between a sword and a glaive (and aren't worrying about a shield) then the functionality is good enough for that, buggy as it is. While equipping the sword, simply drag the glaive to your toolbar wherever you wish - near the beginning of the Items toolbar will keep it handy on your main view if this is something you want to do frequently. You can then use the icon on your toolbar to swap your sword for your glaive, and as long as you don't then swap your melee weapon again, you should then be able to use the same button to swap back to your sword. Each swap, correctly, costs an action though is still instantaneous outside combat.
