Because it is, somehow, an actually very well made game. One of those things where I first just learned about the anime and just thought that sounds dumb, but man the game is actually fun. Also the recent spinoff anime series (cinderella gray) was actually just a very good sports anime.

Umamusume is a gotcha game

Small nitpick, but it’s actually “gatcha” or “gasha” because it comes from the Japanese word “gatchapon/gashapon”. The word is derived from two different Japanese onomatopoeias:
Gasha - The sound of a toy capsule dispenser handle being cranked/turned.
Pon - The sound of a toy capsule landing in the output slot of the machine.

Basically, you know those little coin-operated toy capsule dispensers that you can find in arcades? The ones that have little toys, stickers, candy, etc. inside? They usually look something like this:

Yeah, these things are wildly popular in Japan. They’re colloquially referred to as “gatchapon”. There are massive stores full of these gatchapon machines. Brands will do promos for new anime, TV shows, band album releases, etc… Collectors spend a lot of money to get the rare collectibles from these machines, because not all the toys are the same rarity.

And a gatchapon game is the same basic concept, but in a digital format. You get pulls/draws/{whatever the game calls them} via some method (usually purchasing them, because that is usually how the game makes money), and then those pulls are used to get new things. Sometimes characters, sometimes equipment, sometimes new outfits, etc… It’s literally gambling, because the best stuff is virtually always gated behind some hilariously small jackpot odds.

Again, small nitpick. I just think it’s interesting (and horrifying, because it’s literally slot machine style “keep rolling cuz the next one may be a jackpot” style gambling) how much the mobile game market has come to rely on gatcha mechanics in recent years. There is a lot of (well deserved) condemnation of loot boxes in kids games, but somehow gatcha games have managed to skirt around it.

Thanks for this invaluable insight into the epidemiology of horse girls.