Ah. I hadn't noticed the pattern, thanks.
I was trying to work out how many crates you'd need to open for the rarest items and I suddenly realised I wasn't factoring in the gems, because at this rate (420 crates opened and no drops for some apex and legendary items) it looks like it will be cheaper to buy them with gems than keep trying for a drop.
Although in this case cheaper is still very expensive, especially for apex rewards.