I suspect "shower curtains for days" A/B tests better than anything else they have to test it against. Not that it's the absolute best thing, just the best thing they can automatically suggest. A bath mat that's a color/pattern match for that shower curtain, for example, might convert really well. But one that's not will convert poorly.

I am surprised, as you hint, that they can't tell "one time" versus recurring. Where it should pitch me AA batteries I bought 6 months ago versus the more recent shower curtains, since I will likely buy the former again.