It turns out that to plan a garden, you have to think in 4D: not just how much space the plants take up and how tall they'll get, but how they'll grow over time. And it's not so straightforward because they affect each others' growth: plants in the shade of other plants will grow differently than if they're in the sun, etc. Vining plants can crowd out and stifle other plants. And some plants improve soil quality around themselves, so they can boost the growth of their neighbors.
Ultimately you're making a home for a bunch of living beings who don't move by getting up and walking, but by creating more of their own bodies in the direction of the resources they need.
Maybe I've been sitting in the forest too long, but gardening remains fascinating to me.