If someone is saying it ONLY to make someone give him money it is not good enough answer. My take on this is that a lot of those people just slap "Open Source" on their product to catch people who can invest and care about open source.
If someone does not really care about open source but is just throwing statements around to look good and get money from people, that is not the right way.
Assuming the product is actually open source, the important question to answer is the same as if it's closed: How do you plan to make money? As to why it's open/closed source, any answer just leads back to "and how will you make money?" Neither licensing model is inherently going to make money.
But I have to agree, an answer like "because 'open source' attracts investors" is a red flag. Investors are the means to an end, not the end itself.