Here's the thing.
I'd spend the money to buy engine upgrades, but I am not spending $60 on a Falcon when all I need are two upgrade cards. I'd spend money on c3po, but not $100 on a CR90 that has nothing else I want in it.
They either need to sell singles, reprint on-demand cards often, or always have stock of every ship.
As it is, they're losing money by not selling singles.
Really... they have to reprint them constantly because they are out of stock... where is any benefit to making a card pack.. they sell out all the old stock...