More often than not, this is as a lack of conviction on the investors' part. An investor is either convinced by the founder, or not. They either believe, or not.
In life and in investment, either something is for you or it isn't. "Come back later when there's social proof" doesn't answer the fundamental question of either you're in or not. What it does show is a lack of conviction.
I expect my founders to have conviction on their course of action and how they build their startups. Founders should expect their investors to have the same conviction.
Mark Suster wrote a similar post on leadership more generally. Most principles can be applied to the above