Answer :
Because they feel that separation of power is necessary to prevent the government from becoming a tyranny.
Indeed it's true that separation power only increase the time of the policy making process, but if the one that make the law and interpret the law is the same, the government could technically violate any law and change it afterwards so it wouldn't be considered as 'crime'
Indeed it's true that separation power only increase the time of the policy making process, but if the one that make the law and interpret the law is the same, the government could technically violate any law and change it afterwards so it wouldn't be considered as 'crime'