(Yes, there are always exceptions - like angel.co - and hacks like getdropbox.com also work).
But perhaps the tradeoff of retaining your "perfect" .io domain is not worth the extra friction of convincing influencers that you are credible company, when an hour creative brainstorming session for a .com might solve the problem?
Sure, if you have a hockey stick growth chart growing revenue 100% month over month, it doesn't matter, but it's rare that most startups have that right out of the gate. So why add to your burden?