This has come up in the past with classic quizzes acting the same way. If I remember correctly, the justification for no partial credit is that students can just select all choices and not face any penalty. It would require a mechanism for awarding credit for a correct answer while still subtracting points for incorrect. It's just a broad range of desires for these items and Instructure's stance has been to avoid it.
There are two solutions I offer my staff.
If you use Classic Quizzes, you can install James Jones' fantastic QuizWiz modification on the account which gives you the option to allow for multiple answer partial credit. A button is added to the SpeedGrader that will give points for correct answers. We have this enabled for our staff.

If you're not an Admin, James' blog post has instructions on how to add QuizWiz using a Chrome extension that works great for individuals.
You can manually add partial credit for New Quizzes, but getting to the SpeedGrader is awful. To do this, go to your Grades and then find the assignment column. Click a student score then click the drawer icon. In the detail popout, click on SpeedGrader. That will open the student attempt in the grader and you can score each item there.
Neither is great, but they work.
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