If it were easy for their customers to simply download and save all the movies they're interested in over the space of a month, and then unsubscribe for a few months until the next time they see movies they're interested in, then the entire model would break down - less revenues received, and more money spent on bandwidth per month.
That theory is already debunked. If it were true, Netflix and all the other streaming services would have already failed. Since it is tremendously simple to just hop on over to TPB and grab whatever you want in whatever quality you want it in. People want to pay a reasonable price to have this stuff available legally.
If the human brain were so simple that we could understand it, we would be so simple we couldn't.