If a card costs x money, and operating it every year/whatever costs y money in electricity, and y >> x, it makes sense to mostly talk about the amount of electricity you are burning.

Because if some card with more FLOPS comes available, and the market will buy all your FLOPS regardless, you just swap it in at constant y / for no appreciable change in how much you're spending to operate.

(I have no idea if y is actually much larger than x)