Why is cycling not an activity that can contribute data

See this explanation→Track your cardio fitness levels - Apple Support. Specifically:

Cardio fitness is a measurement of your VO2 max, which is the maximum amount of oxygen your body can consume during exercise. Your cardio fitness level is a strong indicator of your overall physical health and a predictor of your long-term health. Your Apple Watch gives you a cardio fitness estimate by measuring how hard your heart is working during an Outdoor Walk, Outdoor Run, or Hiking workout in the Workout app.

If your cardio fitness levels are low for your age and sex, you'll receive a notification. If your cardio fitness levels continue to remain low, you'll receive additional notifications.

Most people can improve their VO2 max by increasing the intensity and frequency of their cardiovascular exercise. Certain conditions or medications that limit your heart rate might cause an overestimation of your VO2 max.

VO2 max is specifically designed for walking and running; it is not defined for other forms of exercise.

Here is more information→https://www.verywellfit.com/what-is-vo2-max-3120097

Studies have found that the VO2 max measured by an Apple Watch overestimates the results when compared with the described test in the link above, as it doesn’t measure oxygen uptake.

Because the medical professionals who designed the VO2 max test designed specifically for walking and running. It is meaningless with any other form of exercise.

And the actual required variables are heart rate, time, and OXYGEN UPTAKE, which your watch can’t measure; it makes an attempt to by extrapolating from heart rate and oxygen levels, but it doesn’t compare well with a test that also measures oxygen consumption.

If Apple were to do as you suggest they could not get approval to call it a cardio fitness test, because it wouldn’t be. You cannot ride a bike continuously at your maximum exertion level, which is required for a valid test, unless you are riding uphill continuously for 20 minutes at your maximum exertion, such as riding up Pike’s Peak.

The Activity app gives you plenty of information about overall fitness.

The definition of VO2max is not workout specific. The original definition of VO2max defined by Hill and Lupton in 1923 as the oxygen uptake attained during maximal exercise intensity that could not be increased despite further increases in exercise workload, thereby defining the limits of the cardiorespiratorysystem. Thus estimating VO2max can be done also based on cycling activities (although somewhat more complex). It's also a fun-fact that labs that actually measures VO2max quite often uses an exercise bike for the workout part when performing such a test.

But what I do not understand is why Apple hasn't adopted a similar approach as Polar that estimates VO2max

from their Fitness Test - tracking your resting heart rate and heart rate variability and combines that information with your personal metrics, such as age, and then estimates your VO2max from the Polar Fitness Test algorithm.

I record cycling with my watch on an almost daily basis and also record it using a 3rd party app (Cyclometer) on my phone, and I have for at least 10 years. Cycling is not excluded from functions on the phone.

On the watch open the Workout app (green icon with running figure), scroll to Outdoor Cycle or Indoor Cycle (as appropriate) and tap on it. The Outdoor Cycling tracking smart: It will stop and start recording as you stop and start cycling)