Why do I never get DOMS (delayed onset muscle soreness) from cycling but I do after walking? I'm in my late 30s but never in my entire life did I get delayed-onset muscle soreness (DOMS) from cycling. But even walking at a slow-moderate pace (less than 6 kmh / 3.7 mph average) for 2-3 hours gives some DOMS.

I climbed 20% percent hills going all out, I went on long rides at moderate pace, I went on short, full throttle rides, I tried climbing the local hills in the hardest possible gear, I tried climbing standing for 10 minutes straight, I am capable of maintaining 95% max heart rate for 20 minutes and 85% max heart rate for one hour. Max heart rate of 182 was measured by monitoring it for a few years and I'm able to reach it when sprinting at the end of a hard climb. Never, not once, did any of that result in even the slightest amount of DOMS. If it matters, I currently weight 70kg at 1,74m (154 lbs, 5'8) which is stable for a few years.

When someone mentions DOMS in the context of cycling, to me it's like saying 1+1 doesn't equal 2. Please enlighten me.
Let’s look at the definition of DOMS:


  Delayed onset muscle soreness (DOMS) is the pain and stiffness felt in muscles several hours to days after unaccustomed or strenuous exercise.
  https://en.wikipedia.org/wiki/Delayed_onset_muscle_soreness


If you cycle all the time, but rarely walk, then walking more than typically (e.g., a couple hours) could result in DOMS. Cycling and riding use your leg and hip muscles differently, especially in regards to range of motion. Cycling is a very constrained range of motion, compared to walking, especially when it comes to the posterior chain.

DOMS manifests when you use your muscles in ways you are unaccustomed to, this can mean what muscles you use and/or how strenuously you use them.  

This is why I suggested in comments to ride at a high intensity for a period longer than you are accustomed to if you wanted to experience DOMS from cycling.