I used to think so too. Then I read the research on this, and the evidence suggests the opposite of what you wrote: your time is better spent doing retrieval practice than creating your own cards.
I think the counterargument is more along the lines of: Downloadable decks will have a fairly high percentage of irrelevant cards for you, and will always lack a lot of useful cards for you (the latter is something I'm pretty sure is true).
When I am exposed to some new thing, parts of it will stick immediately, and other parts will need some kind of processing on my part before I internalize it. When you are exposed to the same new thing, those parts that stick and those that need processing will usually differ from mine. Hence it's hard to make a deck that works well for the majority.
