To you and me, it's probably more painful settling down for a long flight and realizing there's no AC socket for your laptop, than hearing there's a crying baby behind you. It's not stinginess that's withholding the juice, it's physics.
Simply put, the plane would be too heavy to take off. So claims various readers of The Atlantic, who wrote in after adding up the math and coming to the conclusion that to offer 120V AC sockets to every person you'd need "the equivalent of ten or so Chevy Volt batteries, and an inverter the size of a large refrigerator." This is if every person was using 2 amps of power over an eight hour flight, so it'd obviously vary.
Even a spokesperson for Virgin Atlantic got in touch, who said that for every three-person row of seats, two outlets are on offer for a maximum of 225 watts—though some have complained about the power cutting out. The discussion is a very interesting read for anyone who's ever fumed over not being given a power socket, so I'd definitely recommend taking a look at the (often conflicting) arguments. [The Atlantic]