The first thing you can do is add screws at the backside of each tread where the riser meets it. In your photo you can see the staples the builders originally used, those may not be enough. Add 6 screws (put a screw next to each staple or so) to each length and see if that helps.
The second thing you can try is adding more blocking to help support each stair. You can add a block on either side of the blocks that are already in place in the centre of each stair. If you have some scrap wood laying around you can cut lengths and glue (or use construction adhesive) the blocks. Best to only block every other stair so you can still use the stairs while the glue dries, then go back and do the other half after they’ve dried.