There are plenty of lazy and derivative British sitcoms, but I wouldn't call
The Office a one-off stroke of genius. A great many British sitcoms have similar depth and substance -
Gavin and Stacey or
The Inbetweeners would be good contemporary examples. Going back further, you have programmes like
I'm Alan Partridge,
Blackadder,
Porridge,
The Likely Lads and so on. None of those sitcoms could plausibly have been produced in the US, they're just not sufficiently marketable.
British TV commissioning isn't perfect, but it affords a level of creative freedom that is largely absent in the US, and that is necessary for programmes like The Office to come into being. British commissioners really do think in terms of artistic merit rather than purely commercial value.
American networks are excellent at producing huge international hits, but there's no room in the system for smaller, quirkier productions. The BBC and Channel 4 tend to take a lot of small risks on cheaper productions rather than always swinging for the fences. Success and failure is far less binary when there isn't the pressure to recoup a huge budget. Some of those cheap experiments will be total flops, some will survive as lowbrow primetime filler, some will find a small but loyal audience, a rare few will be huge hits. None of those outcomes are career-ending catastrophes, they're just part of the process. Commissioners expect and allow that level of variance.