Because I can only think of two Brits playing American super heroes in the last few years,so "just about every" strikes me as inaccurate.
Ok, this list will include villains too, but here are all the ones I can think of...
Christian Bale - Batman/Bruce Wayne
Liam Neeson - Ra's al Ghul
Cillian Murphy - Scarecrow
Alfred Molina - Dr Octopus
Patrick Stewart - Professor Charles Xavier
Sir Ian McKellan - Magneto
Vinnie Jones - Juggernaut
Ioan Gruffudd - Reed Richards/Mr Fantastic
Ray Stevenson - The Punisher
Colin Farrell - Bullseye
Dominic Monaghan - Chris Bradley/Bolt
But the operative terms are "American Superhero" and "Brit actor" So the supervillians are out. Non American characters are out (Like Ras al Ghul and Magneto) As are Irish actors Cillian Murphy and Collin Farrell. So that leaves
Christian Bale - Batman/Bruce Wayne
Patrick Stewart - Professor Charles Xavier
Ioan Gruffudd - Reed Richards/Mr Fantastic
I've never even heard of Bolt (must be a C lister), but lets toss him in too.
Dominic Monaghan - Chris Bradley/Bolt
As opposed to:
Robert Downey jr Tony Stark/Iron Man
Don Cheedle- Jim Rhodes/War Machine
Brandon Routh-Clark Kent/Superman
James Marsdan-Scott Summers/Cyclops
Kelsey Grammar-Hank McCoy/The Beast
Halle Berry-Oro Munroe/Storm (I think she's half Amerian)
Ben Foster-Warren Worthington III/The Angel
Chris Evans-Johnny Storm/The Human Torch (and Steve Rogers/Captain America)
Jessica Alba-Sue Storm/Invisible Woman
Michael Chiklis-Ben Grimm/The Thing
Tobey Maguire-Peter Parker/Spider-Man
Ed Norton-Bruce Banner/The Hulk
Ben Affleck-Matt Murdock/Daredevil
Nic Cage-Johnny Blaze/Ghost Rider
Billy Crudup-Jon Osterman/Dr Manhattan
Jackie Earle Haley -Walter Kovacs /Rorschach
Patrick Wilson-Dan Dreiberg/Nite Owl
Jeffrey Dean Morgan-Edward Blake /The Comedian
And thats not counting NonBrits (like our Canadian cousins) playing American superheroes