Your refrigerator sucks the moisture out of these vegetables. There is not enough humidity. In the grocery store and distribution warehouses, these items are often packed with shaved ice or continually misted with cold water.
Try this technique to best preserve lettuces, spinach, and all leafy greens.
1) Wash greens in COLD (even ice cold) water. I add ice to the water.
2) Spin dry in salad spinner or pat dry with paper towels
3) Place cleaned and dried greens in a plastic bag or air‐tight container. I add a paper towel to keep the leaves away from the plastic.
If you ever get greens that you think are too wilted, try reviving them in an ice bath. Fill a basin with water and ice, dip the greens in there, and spin dry. They will crisp up again.