Because they don't allow deeper integrations maybe? I still don't have a watch face layout I like.
When Apple uses private APIs that are forbidden to developers on the App Store to compete with them it's not exactly fair.
So I wouldn't say it's a failure that they don't do that even more often.
Oh and while I'm here the single layer non editable menu / weird grid is also the worst. I grew up texting under the desk on a nine key and only checking after I'd selected the contact to send to. Give me that level of muscle memory again someone, anyone, please.
But in this case at least, the third-party developer has produced exactly the wonderful result they're looking for. The screen shot at the end showing the difference between Apple's map and theirs is so stark and compelling. If I were hiking I'd pay $20+ for their version.
Edit to add: throwing out a price like that made me go check to see what they actually charge, and either Apple's presentation of in-app purchases or their use of it is sad: it gives the same "premium" item like eight times, with different prices. Maybe that's per month and then longer periods with bulk discounts? Maybe they have a lifetime option for $40? If I were a regular hiker, I'd go for that.