Its simple. Consulting companies have to talk to people to get jobs. Those people commonly ask them to sign an NDA. You do it or you don't get the first conversation.
There's an easy solution to this problem: just have your own confidentiality agreement (which you can have vetted, once, by your lawyer), and offer it. We'll sign other firm's paper, but we won't do it without legal review, and we'll avoid legal review if it looks like no work is going to come from the discussion.
Was a consultant, signed way too many silly NDAs.