Because he can draw from personal experience and actually know what he is talking about.
Anyone who is not being condescending towards McNabb just because they can ought to be able to see some parallels between what Mcnabb went through in Philly during his career regarding his receiving corps, and what has gone on in the beginning of Cam's career. McNabb is just expressing his concern so that the Panthers will hopefully take a different path, and head this problem off at the pass by surrounding Cam with quality weapons and play makers so that Cam (and the team) can have the best chance of getting over the mountaintop.
McNabb is not stupid, and neither is Cam. Don't think that Cam is another RG3---all in love with himself and a media darling---too big to talk/listen to McNabb who turned out to be spot on about RG3.
McNabb was highly talented and carried the Eagles on his back offensively for years without a real threat at WR. Cam should not have to be in that situation, and that's why it's imperative for us to draft some offensive play makers at WR who can help maximize Cam's talent.
Problem is (and everyone secretly knows this): Cam is so big, strong and talented, he/they can get away with placing almost anyone around him for a decent offense. And that's what McNabb is concerned about of course. He knows this to.
If you had placed RG3, Kaepernick, or even Russell on this Panther team last year (or Cam's first two years), anyone would know what I'm talking about and see the difference. The offense would have been abysmal.
Nonetheless, I think Gettleman appears to be striking a balance between getting Cam useful receivers and potential weapons, while not breaking the bank. He knows Cam can make many a "useful" NFL receivers great, as he did Ted Ginn last year (and a previously non productive Smitty when he arrived to a lesser degree). Let's see how the draft breaks out as well?
The fact that GMan's getting many tall receivers, reinforces the simple notion (though lost on many of the more excitable Huddler's during games), that Cam can use taller receivers to throw to, as he did in his rookie season with the two tight end sets, since he's 6'6" to begin with.
Of course Competent/NFL level receivers will catch many of those "high balls" that some of those in game Huddler's don't realize other NFL receivers catch on a regular basis as well. So Panther receivers can be improved with out acquiring any "studs", since the Panthers receivers were so bad to begin with last year. However, let's hope one of the free agents, Brandon Williams/McNutt, or Panther draft picks, turn into "break out" status next season.