This is a very good point. One of the most common questions that startups get is "why can't big company Y do X?" and yet time and time again startups succeed in such a space. Of course there are cases where company Y does do X, notably Amazon, Twitter and Facebook. So I think the question or answer needs to reflect the history of the particular companies in question.

And even then. A lot of times Company X does implement Y and it doesn't kill or even slow the startup down. There are probably many reasons behind this. Fundamentally though I think people should just stop asking this question. Or not treat it as such a big deal. Because it's not in the grand scheme of things.