Because not every one uses a sling, plain and simple, and because we don't know how you prefer to have your sling mounted. There's a number of options out there, the most common being to use a Harris #5 adapter and an Uncle Mike's or Butler Creek QD swivel.
Slings are included in the event you want to use one. Think of this, if you're upset because there isn't a sling swivel, you'd be more upset if there wasn't a sling, right?
It's a budget rifle without extras. That's how it's advertised and that's how it's made.