Because you are 'traveling'. As a traveler, you are being exposed and possibly infected in one area and then traveling to another area where you are exposing and possibly infecting others. You are unnecessarily risking transmitting the virus between communities and unnecessarily increasing exposure to the workers of the grocery store who otherwise would not have had that exposure from a traveler.

To keep to the topic; I would question any hike right now as most places are still at the start of the curve. The more we adhere to Stay-at-Home, the quicker we get a handle on this thing.