Why is a "head and shoulders" stock pattern so significant to some investment experts?

the might depend on what information they rely on

Because the human brain likes to create patterns out of any sort of useless information. It's like seeing Jesus on a loaf of bread. It is / was a complete nonsense "pattern" until bots came along. Humans create AI in their own likeness, and bots have come to understand that there are some "technical trader" humans out there that will buy on the downside of a random pattern that they call head and shoulders, as well as multiple other nonsense patterns, and the bots trade accordingly. Now that bots do it, there is some reason to consider it. That said, there's still no logical reason for it. Fundamentals traders make money if they're good at analysis and spend the time. "Technical traders" are get rich overnight people that are just gambling and don't want to put the mental effort into understanding what's happening. You can think of technical traders as Blackjack players and fundamental traders as Poker players. It's always a gamble, but there's some logic and skill to Poker.

see https://youtu.be/mTAiIhU35gE

It's just technical analysis.

Head and shoulders just means there's a chance that the stock could go lower because the "shoulder" makes a lower high than the "head". If it's a false head and shoulder pattern it will break above the "head".

All it is is technical analysis and probabilities that one thing will happen over another.

And what A.J. said, it's for shorter term traders who use TA and not investers.