"Why is it that digital media purchasing and ownership isn't the same as purchasing and owning the physical media?"
Because Sony doesn't have the right to permanently sell you the content, that can only be done by the original rights holder.
So when Sony "sold" people every season of Mythbusters, they were limited by their contract witb Discovery. Once Discovery altered that contract, it becomes illegal for Sony to keep distributing it.
For physical sales, there's the "First Sale Doctrine":
https://en.m.wikipedia.org/wiki/First-sale_doctrine
Rights holder produces physical product. Books, movies, games, whatever. Bulk sells them to distributors, who then have the right to sell it to retailers, who then have the right to sell to you, the general public.
And then you have the right to do with it whatever you want.
There is no digital first sale doctrine.