Because it's the law.
A mile from the airport isn't that far. I'm a couple miles from the airport and my ceiling is 300 feet IF I get temporary approval to fly here at all.
https://app.autopylot.io/#/map
Find out if you're restricted below 400'
First off, your ceiling should have been 400 feet.
Second, the obstacle avoidance should have been turned off for the RTH since the drone can't see obstacles.
The drone OA needs to "see" to be able to detect obstacles. Without the contrast of...