snark... whole lots of butt-loads of snark... that's why i love British comedies... and PBS for showing them while i was growing up...
i would watch things like Fraiser, Full house, Roseanne, Married with Children, etc. etc. ad nasum... and they all seemed the exact same: typical -insert lifestyle here- with some wacky hijinks... cue~ ._. they were funny a few times but the humor always seemed more dumb, and while i do find bits of Big Bang Theory and How I Met Your Mother funny they all seem to devolve into the same hash-browns that every other sitcom has done before... are American sitcoms extremely inclusive, pull the audience in, and have them just laugh at nothing-attached-hijinks? yea... they do... and that's why they're boring... you can put whatever you want on hash-browns, they're still hash-browns... better then tofu, but still hash-browns...
while the british comedies i watched as a kid were hilarious then, and still are now... like Are You Being Served? 'Allo 'Allo, Waiting for God, Keeping up Appearence, etc. maybe it was just the 80s-early 90s brit-coms that i love, but i would rather that kind of humor... snark, snark, and more snark... everyone undermining everying... some social-political satire with everyday random niche knowledge... the joke was funny in itself, then there's about three other ways to interpret the joke and each one is funny along with one that stings, another that cuts, and one that's sad... its multilayered and makes you think... you can be a sociopathic asshole and just laugh at the cruelty or be sincere and have the old woman with her walking stick beat you upside the head for being a pansy (I absolutely LOVE Diana from Waiting for God

) though i should probably check out some others, and they usually are rather short... think the last brit-com i watched was 'Mongrals' and loved it... and hated that it was only 2 seasons ._. *shakes fist at the BBC* muragh!!
suppose it is a perspective thing... OP for all the reasons you like American sitcoms over British ones are all the reasons i dislike them, and all the reasons British sitcoms are 'morose' compared to American ones are why i like them... a subtle dark humor without being bluntly stated as dark humor... muhahahaha *goes and watches Waiting for God while cackling along with Diana*