I’ve tried to rationalise a reason (not saying it’s the reason WK don’t have an undo feature, just thinking of any reason) and I’ve thought of maybe two plausible reasons.
You learn about WK being a great tool for kanji learning, so you start using it trying to learn kanji for an upcoming exam (JLPT or something). You, naively, make liberal use of the undo feature “oh I was close enough” “I was tired, I would have gotten it correct if I was well” etc etc.
You get to a level where you’re supposed to know all the kanji for N5, you take the test and flunk it. You didn’t actually know the kanji because you cheated yourself out of the learning.
So off you go around the internet and telling everyone how bad WK is, they take your money and don’t teach you anything. They don’t offer what they promise, blah blah. It’s just bad avoidable PR. Plus you lose that person’s sub, and possibly future ones.
(WK could lose subs removing 3rd party script functionalities too, I know)
The second reason is pretty much related to the first. By not having all these extra features, you have a ‘fool proof’ system. It works one way, and one way exactly (which I know isn’t going to work for everyone). Beyond opening a second window and searching for answers you don’t know, there’s strictly no way of cheating. You’re using WK exactly as intended, using only the features it has, you’re guaranteed to experience WK as the devs developed and also the same system that everyone else does. Vanilla WK is very singular in its approach. You’re given a prompt, you answer it, it then checks if it’s right or wrong. There’s no ambiguity. “Am I allowed to use undo here? Will I be cheating myself?” etc
WK can then advertise that they have this system, and it works exactly this way, and does exactly this blah blah blah They don’t have to worry about 2 users having different experiences (beyond ability level and speed of completion)
(The discussion about ambiguity and the choice of radicals/mnemonics/on vs kun/primary meaning/etc is beyond the scope of this post)
Whether or not I actually agree with these, I’m not going to say, it’s not that kind of debate. I’m just trying to present a ‘non’-biased (I’m not pro WK but naturally their reasons are going to be pro them) discussion as to why WK doesn’t have a native undo button.