It kinda makes sense. I mean, technically you like your own post, otherwise you wouldn't upvote it.
But in practice, it does make sense. If all posts are automatically upvoted, there won't be any taboo or ill feel about users upvoting their own post as everyone does it.
You can upvote yourself on most platforms. You can like yourself on Facebook, you can give yourself a heart on Instagram. It's just not considered very tasteful as it is off by default.
My guess would be that it's kind of pointless to implement checks if the liker is also the poster just to prevent them from liking themself.
I believe in many elections (in the US anyways) you can vote for yourself. I assume most people always do. I think the idea of not supporting yourself is maybe a little odd.
There've been a few occasions where I disagreed with my own post. I couldn't just let myself get away with bs comments, so I had to downvote to teach myself a lesson.