"These two different movies are tonally very different and releasing on the same day! How quaint! You should see both of them back to back and purchase more tickets!"
The "Barbenheimer" meme came about organically*. Two movies based on an extremely recognizable IP and a well-known historical figure** respectively with stacked casts and popular directors released on the same day. People were independently interested in one or both, the meme started from there.
Movies of different genres come out the same day all the time because that's good business. If you're releasing a superhero movie, do you want to run against another superhero movie or a romantic comedy? Which one is going to eat into ticket sales more?
Expect a lot of this in coming months. Gonna be exhausting.
*Both studios definitely rushed to cash in on the trend though.
**Also a very controversial figure, but I am not going to say more than that because its beyond the scope of this post.