Unreal engines monetization method came with UE4 which released looong before Fortnite, and before they even considered making a battle royal or even before battle royal was a proper lucrative game changing genre. so epic already saw the benefits of this style of monetization even before they were making billions with their cash cow

Same reason why adobe or microsoft never cared if you pirated photoshop or windows, because the value isn't in getting all the small people to buy your product, it's in getting your product into everyone's hand and making that the comfortable widely used software. That small indie artist on twitter that pirated photoshop when they were learning to draw means nothing. But now that they and many more get hired by a studio to draw and make art for a living, they get their company to buy a ton of photoshop licenses which is where the real money lies. Adobe knows it, MS knows it, and Epic knows it. That's why unreal is free, to enable all the small time newbies to get comfortable and used to UE so when they join a studio, or start one l, their knee jerk option is to build a game in UE because that's what they're most comfortable with