Why can physical changes separate the components of a mixture but not the components of a compound

Why can physical changes separate the components of a mixture but not the components of a compound?

Several types of mixtures can be separated by physical means. Heterogeneous mixtures imply the different parts are not equally distributed, much like how on a pizza the toppings are not located in the crust. A homogeneous mixture is one where the components are equally mixed, such as orange juice or lemonade (excluding the pulp).

A common heterogeneous example is to separate oil from water by placing the mixture into a separation funnel and draining one level of liquid out. You then have the two separate components. Basically, if you can see the different layers in a mixture, you can physically remove them by either picking out the solids (either with your hands or running the mixture through filter paper) or for liquids using the funnel.