This is taken from the article I provided:
You can calibrate your Apple Watch to improve the accuracy of your distance, pace, and calorie measurements. Calibrating your watch can also help it learn your fitness level and stride, which improves accuracy when GPS is limited or unavailable.
Doing things shown in the article, such as adding your height or age, or enabling location services all helps your Apple Watch improve its accuracy.
This is a user support forums so none of us work at Apple, so we won't know the reasoning behind Apple's decisions. I would suggest leaving a feedback report to Apple about editing swim workouts.