Well, both require making land available, and while amenity gardening is manageable, a botanical garden is often a research site that requires a lot of expertise to set up and maintain. Mind you, that's just fine, the county can and should hire people.

Looking out my window, I see trees planted on the sidewalk along a road.

It takes little space away from pedestrians, but provides a lot of shade. That seems both easier, cheaper and better that taking up whole city blocks.