I would assume its because header flanges warp, regardless of thickness.  I've seen cast iron manifold do it too.  I've had times where I had to rig up a spreader to get the bolts in the holes in the head.  The beads small surface area actually ensures a better seal than trying to conform the entire flange to the head.  If good gaskets are used, and the bolts kept tight leaks are rare.  I prefer solid copper gaskets for headers.  Great seal, and if they do develop a small leak just tighten the bolts and it goes away. 

Cab4word67 wrote:

I used the no leak  aluminum ones that look like a bunch of thin metal sandwiched together on my ZZ4 SBC and have never had a leak. 
But on my built 230 inline 6=8 even with the bolt on the rear of the cast manifold I would loose a gasket and ended up sealing with silicone. Never failed me after that. they say a six can warp at will on the end two ports

Its a really long manifold to keep straight.  Dodge Cummins engines had this issue and the aftermarket developed manifolds that have slip joints in them to allow expansion and contraction of the sections without hurting the gaskets or cracking the manifold.  Pretty slick solution really.