there was a time Apple announced product and it was ready at launch day. Nowadays Apple offers unready product with no main feature.
This isn't just any normal feature. When you make something that can be considered a medical device, it falls under a regulatory umbrella known for being very slow. If they held onto the feature until every single country's protective regulatory agencies approved it as a medical device, it might take several months or even a year or longer, and that would be absurd for a consumer device that's getting updated so far every year.