As a silencer designer, I can tell you some reasons:
--The extra volume behind the muzzle is overrated. It doesn't help as much as you think. There are other factors I can control forward of the muzzle that will have greater impact.
--It makes the can heavier without a proportional improvement in sound reduction.
--I'm designing for something outside my control. There will always be someone asking for that real estate behind the muzzle. Gas blocks, rail systems, barrels with large OD, varying muzzle device cross-compatibility, ect. There will always be someone asking for some variant or mad that it won't fit on their weird one-off rifle.
Don't get me wrong, there are positives, but they're very specialized in relation to the rest of the can. I love my Ops Inc 12th model (AEM5 now). It's phenomenal. It's the quietest 556 can I've ever sound tested (muzzle and at-ear). But, it has the tightest bore of any can I've ever seen. It can do that because of the mounting method and it relies on that bore to work as well as it does on that SPR rifle platform. If you opened it up it would sound, well, like a Surefire.
I've never seen a military one without a cap strike. (not all cans use the two point method AE does, so there's little benefit there to any AAC, Surefire, etc.). I've metered our Sandman-L within one dB of the 12th model with similar awesome at-ear results and it has a bore that is over .100" larger in diameter. Closer to a full 1/8" at the front cap. It also weighs less and can be used on 7.62 as well.
Having said all that, I'm sure I'll design something someday with some behind-the-muzzle volume. But it'll be balanced with a really well designed baffle stack and it'll also invariably be a very specialized suppressor.
Todd Magee
Dead Air Engineering