For the same reason farmers don't own grocery stores; making a film is different from distributing it. The filmmakers know how to make a film but they don't kno

For the same reason farmers don't own grocery stores; making a film is different from distributing it. The filmmakers know how to make a film but they don't know how to release it to movie theaters and actually receive payment from ticket sales, heck they first have to actually convince the movie theatre to show the film in the first place. They also have to get it classified and rated and sometimes even censored, get all the appropriate music licenses for different markets, advertise for it including making trailers and posters and that work often has legalities associated with it all on its own, and the host of work that is needed to make it worthwhile to release a movie in various foreign markets. The filmmakers release it in their local market first, usually the U.S. is the biggest market. If the film does well and an UK distributor has the confidence that it's worth releasing in the UK then the filmmakers will sell the rights to the distributor to handle all the stuff I mentioned above.