There is definitely this, but also: if it looks "refined", people start getting attached to what they see, and it affects how they react to the final product.

Any change from that haphazard throwaway with nice colors is suddenly a change they have opinions about, because it feels like a change.

If you show them something that's obviously not what will ship, they don't get as attached.

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This is also partly a "most people don't understand the design process" thing, and just how much reworking and restarting is generally necessary to get an actually-good end result. If they see hundreds of mockups (or even sketches), they'll wonder why you haven't made hundreds of products, rather than those being merely tools used to think along the way.