I imagine it's because a few bad apples out there would use this to scam the system and ruin it for the rest of us.
Being completely blunt about this, it defeats the purpose entirely. A creator shouldn't be wasting their time earning coins that way to pay themselves. They should be more focused on creating content to inspire readers to pay them in tips.
If you cut out the readers tipping creators part of it and just have creators tipping themselves, then what's really the point of creating comics? You can just watch ads all day and earn money pointless that way.
Not to mention there's a limited pool of ads to watch from. It's much more preferable to have readers pulling from this pool of ads than creators that are just turning around to tip themselves.
Also it'd open the system up to bot attacks. A few really really bad apples can create bots to watch the ads and the bots tips their maker, and that will drive the worth of the ad revenue down. From every angle, it was a smart decision to make that not available.
I get the immediate appeal of tipping yourself but it would not help the feature or the community grow.