The cold truth is that we don't. We only assume this to be true because of the semantic nature of the verb. So we should be cautious to distinguish solid facts from these sorts of unverified or unverifiable assumptions. I mean, I don't know about any of you, but personally my sneezes never sound anything close to "pneu".
I've been exploring an alternative origin of this word, not from an echoic origin, but rather as a possible Semitic loan. I've mentioned before in Pre-IE Syncope and possibly expanding the Metathesis rule that there may be some loans from Proto-Semitic that exhibit word-initial metathesis of consonants in PIE after experiencing the event of Syncope. Metathesis is one common tactic for renormalizing syllable structures after awkward clustering is caused by the disappearance of unstressed vowels. Such metathesis is guided by a universal rule in world languages known as sonorancy hierarchy. Certain clusters are universally avoided such as *rp- or *bft- for example. So if it's generally accepted that Pre-IE must have undergone Syncope, then it's naturally implied that awkward clusters like this should have occasionally arose and that there must have been a mechanism in place to restructure these roots.
So in that light, here's yet another hypothetical Semitic loan path to discuss amongst ourselves:
Proto-Semitic *napāḥu "to blow" → Mid IE *napéwa-
This would then become late MIE *nᵊpéwᵊ- (via Reduction) and then *pneu- by early Late IE via phonotactically motivated Metathesis in order to avoid the less desirable outcome of **npeu-.
Enjoy that thought. I know I do.