I used to think like that: exercise literally as a prescription.
Then I bought a bike. Do you remember how fun it was to ride when you were a kid? It is just as fun when you are an adult. As a result, you don't carefully perform your prescribed dose of weekly exercise, you just go out for a ride when you feel like it.
It doesn't need to be a bike. The point is to try a few things until you find something that you enjoy doing. Then exercising won't feel like a chore. Instead, it will be that thing that you do for fun, which just happens to also make you healthier.
Depression is really good at sucking the joy from normally joyful activities. If you need to remind yourself to eat, to sleep, to shower, to change clothes, then “exercise when you feel like it” translates to not exercising at all. Having a minimum suggested routine or target is helpful for the people who are struggling the most.
Then I bought a bike. Do you remember how fun it was to ride when you were a kid? It is just as fun when you are an adult. As a result, you don't carefully perform your prescribed dose of weekly exercise, you just go out for a ride when you feel like it.
It doesn't need to be a bike. The point is to try a few things until you find something that you enjoy doing. Then exercising won't feel like a chore. Instead, it will be that thing that you do for fun, which just happens to also make you healthier.