You're not imagining it—cotton-poly blends are prone to neck stretch because the collar is a separate band of fabric that's sewn on under tension. Every time you pull the shirt over your head, that band gets yanked. Add in the heat and agitation of a washing machine, and you've got a recipe for a stretched-out neck.

Here's what actually happens: cotton fibers are strong but not very elastic. Polyester adds some resilience, but even a 50/50 blend won't snap back if the band is repeatedly stressed. Think of it like a rubber band that's been stretched too many times—it gets loose and stays loose.