"Why do you think automakers
"Why do you think automakers are obsessed with Plug-in hybrids with less than 30 miles of EV range?"
Because that range covers well over 90% of the driving people do.
Larger energy-storage batteries are effectively permanent liabilities; they age (and thus require replacement) and they add weight and size to the car, reducing energy efficiency. Thus there is a point of diminishing returns, where a larger battery extends range but does not make up for the additional fuel/energy used to drive the larger vehicle.
Today adding range to PHEV's is a plus primarily because in most places, charging is still difficult to come by. Thus a 30 mile range really only enables a 10 mile drive when battery aging and lack of charging at the destination is taken into account. As chargers proliferate this issue will become less important, and the benefits of a 15 mile range will start to equal what the benefits of a 30 mile range did before charging was widely available.