Software development is a balancing act. You need to pick and choose not only what features to add, but when to add them. Sometimes, mistakes are made in the planning and you get a situation like this.

What likely happened, is that these kinds of features were deemed less likely to be needed, since the majority of lemmy users will never run into the need of them and there is technically a way to handle the situation (nuking your instances image cache.) But you'll likely see a reshuffling of priorities if these kinds of attacks become more prevalent.