Why didn't Bethesda make it so that when enemies drop their weapons its still considered equipped and still in their inventory so that they clean up properly like in TES4?

I don't know. People have been asking themselves this question since day one. The Unofficial Skyrim Patch did it. You would think Bethesda could have done it too.

Yeah I heard a modder did exactly what I described and am wondering why the thought never crossed Bethesda mind.

I imagine the original concept was to make the world in Skyrim 100% permanent. At some point during development, it probably began creating more problems than it was worth due to bloat. If it was late in the cycle, and the team had already written the code to make characters and items remain indefinitely in the game world, they most likely only wrote out the code for the big things. Bodies contain specific references to facial design, hair styles, armor, clothing, etc. That's a LOT of data all tied in to one "body". If I write out the code for removing bodies, it takes care of all the items attached to it.

But because equipped weapons and shields go flying and become their own entities, it would mean I would have to go through each and every weapon, staff, shield, etc. -- one by one -- and write the code out for each item. There may not have been enough time.

Hey, at least someone modded it in!

For myself, I've never had a problem with bloat, but I also don't use any mods and that add all sorts of new items and what not.