Fortnite lets you pay some real money for a cosmetic upgrade to your character. That's it. There's not even random loot boxes or anything that actually affects your chance to win. Video games have done this for ages.
The only difference is that Fortnite is currently very very popular with quite a young demographic, and that demographic also consists of emotionally immature kids who bully each other over everything. This bullying is going on regardless, Fortnite being so popular just makes bullying about it more visible/obvious.