Why isn't blood oxygen available for free with the Apple Watch?

There is no purchase required, but you have to meet these requirements to use the App, which are stated in the "Related Article."

"Here's what you need

• Make sure that the Blood Oxygen app is available in your country or region. You will be able to see this during the setup process.
• Update your iPhone 6s or later to the latest version of iOS.
• Update your Apple Watch Series 6, or Series 7, Series 8, or Ultra to the latest version of watchOS.*
• The Blood Oxygen app is not available for use by people under 18 years old. You can confirm or set up your age in your Health Profile."

I'm not sure why you think you have to purchase an app to get a Blood Oxygen reading on an Apple Watch? There is no additional cost for the feature. You do have to own at least an Apple Watch Series 6 or newer and live in a country where the feature is approved by the minster of health for that country.

This link may help you --> How to use the Blood Oxygen app on Apple Watch - Apple Support

You are welcome.

No worries at all!