If an Indian reservation lies within a state's boundaries, why can't a state simply enforce its laws on the reservation just like it can anywhere else in the state?

Indian reservations have their own jurisdiction through tribal sovereignty.

Explanation:

Indian reservations in the United States have their own jurisdiction and generally have what is called tribal sovereignty if they are federally recognized by the Bureau of Indian Affairs. This means that in the event of a criminal offense the state government where the reservation is located has little jurisdiction unless the crime that was committed involved non-tribal members. It is a little different in what are called "Public Law 280 states" where some jurisdiction was given to state governments, namely states like California and Nebraska. Many Indian Nations in Public Law 280 states are still without functioning justice systems for criminal offenses because the Bureau of Indian Affairs and the FBI used the law to argue they are not responsible for funding.