One of the ways I've attached the curtain to the wall is to attach a cup hook to the wall and use a drapery pin or even a safety pin in the side hem of the curtain and hook it to the cup hook. You might need to use an anchor if the cuphook is screwed into drywall.

You could use drapery hardware with a French return

Tacks won’t hold because the fabric moves every time the curtains are touched. A small screw-in hook or a low profile bracket does a much better job since it anchors into drywall with a proper anchor. You can match the finish to the curtain rod so it blends in. Once you switch to hardware meant for weight and movement, the top edge stays put.

Change your curtain rod to one suited for the look you want. That rod does not assume the curtain will be attached to the wall...

Don't care as much about the look, just want it to be functional, with minimal effort. Not sure why the installer had done it that way

Late to this party and maybe this will help others. Cup holder screwed into the wall. Drapery hook placed into the wrong side of the fabric. See below.

Using the hook, insert into the screw eye. You can see there is no bunching of the fabric.