because other companies may hold the rights to them.
For example, a Passport subscription will get you most of the shows that have been on Masterpiece for the past few years, but it will not get you the OPTIONAL British and other foreign shows that individual stations can choose to carry or not. This includes favorites such as Midsomer Murders, Vera, and Shetland. (Midsomer is on Acorn, while the other two are on BritBox.) I , Claudius and the old version of Poldark are both on Acorn.
Passport has one huge advantage over the Amazon add-ons. if you subscribe through Amazon, the dramas are one channel (PBS Masterpiece), the documentaries are on another (PBS Documentaries), and the lifestyle and practical shows are on another (PBS LIving). With Passport, you get all three genres for as little as $5 a month. In addition, you can access whichever shows your local channel produces. Twin Cities Public Television makes a lot of documentaries with a local focus, so that's a big perk for me.
I would absolutely love it if there were one-stop shopping for all the British and other foreign shows, but no such channel exists. Your best sources are
BritBox (all British with one or two exceptions)
Acorn (English-speaking countries outside the U.S., plus a couple of foreign language programs)
MHz Choice (mostly European programming, with a few ventures into other regions, in the original languages with subtitles)
ViaPlay (Scandinavian content)
Criterion (classic, foreign, and art house feature films)
Walter Presents (also European content, but most of it is already included in Passport)
Netflix (a lot of foreign shows, with the option of watching in the original language with original-language subtitles in addition to versions with English subtitles)
We live in a golden age of access to the best TV in the world. It's good we're in a golden age of SOMETHING, even if everything else seems to be deteriorating.