Why would that be a bad thing? Both UK residents and those who are not resident in the UK are subject to a two week quarantine from a list of countries at present (along with the inverse) - why not make it universal as opposed to the current system of sticking one's finger in the air and see what happens? See further, the hilarity that happened a couple of weeks ago when Spain was removed from the exemption list.
Gives the hotel industry a boost too as they'll be used for quarantine purposes.
Because it'll do massive damage to the tourism industry and to international trade generally...