I think the reason is that no one (who has the correct leverage / area of ownership / authority) cares.

Big companies have processes for deciding what gets done and which features are prioritized, and if the people in that loop aren't power users of those features, and they don't have the kind of metrics or analysis framework that would indicate that those features are important, they won't be prioritized.

I don't think there's some kind of conspiracy to make search worse... I think it's just that nobody cares enough, and with time the features break as everything else around them is changed