The law of every country in the English-speaking world started that way. Australia's current system of law, for example, commenced as a branch of British law on 26 January 1788. All the laws in force in Britain at that moment were presumed to apply; laws governing institutions and issues peculiar to Britain-as-a-place just weren't of any consequence.
Periodically you go into the collection and clear it out. In Australia in the 80s and 90s there was a law reform movement and as a positive side-effect enormous research was done to discover the true coverage of "Imperial" laws still technically in force. Our various Parliaments passed various Acts to repeal and replace old laws.
For example, where I come from, in the Northern Territory, the Law of Property Act repealed laws going all the way back to shortly after the Norman conquest.
