Feral control alone cannot work. Responsible ownership is required to reduce the number of cats which go feral. Entire males roam in search of mates and may become lost. Unspayed females will also roam if they can smell a tomcat nearby. Roaming cats are attracted into feral colonies where there are potential mates.
Some cats simply stray too far from home and become lost.
Many young cats are abandoned when no longer kittens, partly because they are no longer cute and fluffy, partly because of the expense of neutering. It is important to neuter pet cats to reduce the likelihood of straying and to prevent unwanted kittens which may themselves stray and breed.