Tissues that aren't stressed naturally atrophy to conserve energy, which saves on food, which is adaptive in evolutionary terms. This has the natural implication that those tissues will reach an equilibrium where they are barely stronger than your most basic needs.

If the only stress you experience is getting off the couch to get some food, just doing that will be a little difficult. Larger, sudden stresses will cause injuries because your tissues won't be adapted to handle them.

Exercise ensures that we mentally and physically exceed our normal stress thresholds, so the remaining stresses of life is less stressful by comparison.

I view exercise as giving your body, and brain, proof that the rebuilding is still needed, and information on where to direct resources.

If you never take a walk, why expect your body to rebuild itself to be good at walking? Eventually it'll lose its capacity because it (correctly) "learns" that isn't important to you.