Here’s a good summary of the reasons to keep your cat indoors, from the Nova Scotia SPCA in Canada.
As the author says:
Most veterinarians believe cats should be indoor pets. So does the NS SPCA and other rescue groups. It’s a myth that letting cats roam freely outside is ‘natural’ and necessary. Cats are happy being indoors and don’t need to be outside to have a fulfilling life.
13 May 2017