Design by committee
A camel is a horse designed by a committee.
It's when a designer and their project's stakeholders come together to design a horse, but direction misaligns, opinions conflict and the outcome is a lumpy, humpy mess of a camel.
Collaborative thinking sounds good in theory, but it leads to inexperienced decisions being made. Stakeholders may focus on features and capabilities, instead of focusing on user needs and experience. As a result, the product is less than the sum of its parts and users find it mediocre at best.
This doesn't mean that everyone's ideas are bad. Anyone is capable of a good idea, but the designer knows how to prioritize ideas. They understand that it often takes a hundred ideas to get to a great one. Their "opinions" come from years of refining their designer's eye based on research and experience.
A design can feel like our baby. It's born from us. But that's not really the case. Emotions need to be removed from design work because design work is business. A stakeholder's best contribution to the workflow comes from communicating their needs and expectations.
Designers do the designing, they are the hired experts, afterall.