Why are people not bored of doing ordinary things (like walking, etc.)?

Just like we need novelty to spice up things in life, you need a routine, uninteresting thing for the yin-yang. The good thing about walking is that it is a medium for other things like creativity to happen on autopilot in your brain while you are walking. You get the benefit of two exercises - physical and mental. No two walks are alike on the same path. It could be something that falls on the earth, that makes a day. A dog or a fellow walker or jogger. Some curious incidents. Each day you come to the walk with a different energy. Someday you might take longer than usual or do it fast. If you look at the metrics, you might realise that your stamina has increased and go for longer walks or newer routes. You will realise the joy of walking and realise it is not the walking, its setting the mind free which rejuvenates you each time even if its for a few minutes a day. All that blood pumping in your feet, make you feel accomplished.

On a general note, during a routine is when you are able to problem solve subconsciously. It slows you down and makes space and time for things to think through.