I suppose for those that do, a laundry room would be needed.
My roll-aboard suitcase holds a tremendous amount while still fitting is the overhead compartment on the plane. Most of the cruises and vacations I've gone on have not had self-service laundry facilities available, and it hasn't been a problem. I don't take a new outfit for every day, but rather take things to "mix and match." The clothes I do take can all go with the two pairs of shoes I take. In my husband's case, a little Woolite or the ship's shampoo can go a long way in not needing to pack 10 workout t-shirts and shorts. He buys the quick-dry travel stuff and never has a problem. I prefer not washing things out in the sink, so I just take enough socks etc for the whole trip. And it still fits in a roll-aboard.
If I ever run into a problem on the ship, I can send laundry out, but that's a rarity.