The other reality is that it is incredibly easy to get a bipolar diagnosis. It's the trendy diagnosis. It is also misdiagnosed a lot in women by the old guard of psychology/psychiatry who think women should act a certain way. Bipolar NOS (not otherwise specified) is a catch all diagnosis. ADHD (which used to be a trendy diagnosis) is on the differential with bipolar and has a lot of similarities with a manic episode. The DSM criteria for bipolar includes such subjective symptoms as "risky behavior" which are applied more stringently to women than men.
Is it really that easy to get a Bipolar diagnosis in the US? In the UK at least it is very difficult to get the diagnosis confirmed and people usually have to wait many years to get appropriate treatment.
Perhaps in the US with insurance picking up the tab for treatment they are more willing to diagnose it. More likely as you've said it is a trendy diagnosis (or rather a self-diagnosis). People without knowledge of the condition just decide they have it to explain their behaviour.