In simplest terms, "because that's how the contracts were written before the Internet".
The creators and publishers/distributors of the established industry have historically made greater revenues when they were able to re-sell the rights to their work to multiple territories.
In part, this has even been a good thing at times – once you have a licensee with "feet on the ground" in a given country, you immediately have someone with a financial interest in stamping out piracy of the licensed work.
Similarly, having a company that understands book distribution infrastructure, or television broadcast restrictions in say Chile, can be immensely helpful to a publisher/TV studio in Spain – despite the fact that they share a language (more or less).
Of course, when it comes to, say, English-language products, it's an incredible frustration at times to be an Australian or British citizen waiting for the release of an eBook, album, movie, or TV show from the US.
And lest we forget, there are a lot of countries and languages out there, and even the most popular books and movies never get a proper localized edition… Which again is frustrating for the locals – who may well be fluent in a licensed edition's language, but simply unable to legally obtain their own copy.
It's not an easy system, but it has more or less worked for a very long time, which in turn has created (accreted?) a lot of established players who would faced reduced revenues or even extinction if more "internet-friendly" terms were quickly and universally adopted.
It seems almost inevitable that things will change once digital distribution overtakes physical product sales worldwide – master licensors will have fewer reasons to cut in the locals based solely on geography, and instead opt for a "world-wide per-language" license model, or something similar. Unfortunately, waiting for this to happen is a bit like waiting for cheap efficient solar power – always just over the next hill, always 3 years away.