Why do some clothes shrink when washed in hot water and why can’t they stretch back to their normal size?
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The short answer is entropy. Both natural and man-made fibers are composed of long molecules that are preferentially oriented in the axial direction. Weak chemical bonding between adjacent chains maintains the alignment. Heating is sufficient to disrupt these bonds, allowing the molecular chains to relax into a less-oriented configuration, thereby contracting in length. The temperature at which these transitions occur is called the glass transition temperature, which is different for different materials.
It is theoretically possible to re-stretch the fibers, and maintain them under tension while cooling, to reset the extended length, but practically difficult, once in fabric form. Materials that have more permanent cross-links exhibit much better thermal stability, but they also are hard, stiff, and not suitable for making fabrics.
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