Because even though the V6/7506 are popular studio phones (rugged, affordable, bang for the buck), they aren't actually used as an end-all reference. There are plenty of popular studio phones. And even if they are popular, we don't know to what extent headphones are even used in the recording process as there is typically strong bias towards making things sound ok on boomboxes and car stereos. The reference in question sometimes is average consumer audio for most mainstream recordings. And most mainstream recordings are pretty bad anyways.
Imaging on headphones can be better to comparitvely priced speakers. Room-filling ambience is what headphones will always lack, but they can still portray depth and have precise imaging. Headphones can create a very precise headstage. Yes some headphones have much better imaging than others, as well as wider soundstages. They all can be somewhat lacking in the "ambient" room-filling sensation, except maybe an AKG1000 paired with a sub. Most speaker setups need very ideal room and placement as well as GOOD speakers in order to create any decent imaging anyhow. Many headphone listeners have adapted their listening such that "headstage" is very transparent to them.
So headphones can image better than speakers with less effort and cost. Headphones can't provide room-filling ambient sensations. Some headphones portray sound differently, some being more distant, forward, wider, narrower, etc. I did not consider soundstage on the V6 either a particular strength or weakness, mostly average. There are phones that are better in this department for sure. Yes V6/7506 or other pro phones, AKG240, Beyer 770/990, MAY be used now and then...but IMO probably a limited usage in the process. So even if they use these phones...they don't use these phones for the purpose of making the music sound RIGHT on those particular headphones.