Exercise always seems counterintuitive. After all, things tend to wear out and break when you use them. You don't use the good scissors for most things or they won't be good when you need them. We would be a lot less grumpy if the way to build muscle was to take it easy, and you only needed to exercise if you were overbulking and needed to wear some out. So we want a pill that triggers the regenerative effects of exercise without the actual exercise. Human Growth Hormone without the problems.

Evolution doesn't care if you're grumpy, it only "cares" if you succeeded in reproducing.

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.

Our body is breaking down constantly, but it's also rebuilding itself. It's not an inanimate object which wears down until it breaks.

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.

I really enjoyed the book "Move your DNA". It talks a lot about the loads you place on your body (even ones you don't think about) and the effects that has. Definitely worth a read.

Lymphatic system is drained only through movement. I wouldn't be surprised if the same holds for the one that was discovered in the brain recently.