Because I perceive the cost of memorizing it to be almost zero.
There's of course a need for nuance here. I do write scripts to automate stuff and I do have aliases configured for certain commands, directories, etc. I don't try to remember all the things, just so that I never have to look something up again.
If I wasn't a heavy anki user already, then I certainly wouldn't set it up just to remember some commandline stuff. But that's not the situation I'm in. I use it for lots of things already and I have an established habit to review the cards every day. Adding a card to remember specific flags has incredibly low overhead for me.
A different, but similar example is using Anki for learning library/framework syntax. I made heavy use of it when I first learned Pytorch. There are so many different commands to wrangle your data into the shape you need it to be. I frequently mixed them up and got frustrated because it doesn't necessarily result in runtime errors and then it's hard to debug (because I didn't know what I was doing). So everyday I chucked a couple of the new commands I encountered into Anki and by the end of the week I was comfortable doing all kinds transformations. Would I have learned to do that without Anki? Probably. Did it give me an additional opportunity to consolidate that knowledge for very little cost? Also yes.