Social media policies and professional development

A niggling question: why do companies typically forbid their IT staff from discussing the fact that they’re looking into buying/using/are using a given technology or software?

Are they afraid their staff may learn from others mistakes and miss out on the golden opportunity to make all the mistakes themselves? Is it the fear of learning from others? The fear of the curtains falling and people being seen in their true light - that they didn’t hire the A Team but merely a mix of C and C+?

After all, if staff from Companies X, Y, and Z didn’t already discuss failings of Widget D (and thus likely breaking their organisation’s corporate policy) Company W would’ve been none the smarter and would have ended up with a faulty Widget D as well. End result is generally a much improved Widget D, a much humbler Vendor Q and a richer for experience Companies X, Y, and Z. And financially better off Company W that didn’t buy Widget D, instead opting for Widget S from Vendor P.

So why, again, do I see policy after policy that explicitly forbids such information sharing?