Why do couch cushions have zippers

After about 3 years, the main seating cushion on my couch (leather 2 seat recliner) has sunk in a lot towards the back and its pretty uncomfortable.

The seat cushion does not separate from the couch. Its fixed in place with cloth/staples and maybe some other stuff I cannot see along the sides (framing is metal and wood). I've searched this couch high and low for zipper access to this seat cushion and it just doesn't exist. The rest of the cushioned portions of the couch all have zippers and I can add foam if needed (the back and footrest)

Really not sure what to do. The idea that I had was to cut the cloth portion of the cushion at the back and try to sew in a zipper. This might allow me to add stuffing but I don't think I would have enough space to change out the whole cushion. I don't really know how to sew so I bought one of those handheld mechanical stitchers to practice with. Not sure if I will have enough room or material to do this though since I cant take the cushion off and have space to work.

This is actually a matching chair but was easier to show on this than the couch.

Kind of afraid that this might be too big of a job to DIY. Kinda scared to take apart the whole couch.

Any advice or suggestions would be great.

yes, depending on the material though, you might have to use cold water or let air dry.

Absolutely. What's the worst that could happen?

(other than either the fabric falling apart, or the cover shrinking so much it won't fit a pillow, let alone the couch cushion)

Don't dry them in the dryer or you might not ever get them back on (depending on the material).
Actually, we dried ours in the dryer and they still fit... but I was worried about it.

Yes.. I'd do gentle cycle and open the lid once the tub is full of water, then sorta handwash and let soak for awhile, then close lid and let it complete the cycle. Do not dry, I'd hang them up and point a fan at it for a day or so

You can also buy replacement foam, so if yours is sagging take the clean/dry covers to an upholstery shop and have them cut you some new stuff.

What? Is this a real question? Of course, you can.

That's what the washing tags are for. Yes, you can wash them.

could put them in the bathtub with woolite, manually agitate a bit, then rinse and drain thoroughly. air dry.