Why are contributions to the education of the broader public not valued much within academia?

Scientists not only need to discover new knowledge but also have to share the knowledge to others. For the former, we have things like universities, collaborate groups and journals to take charge. For the latter, we have various forms from teaching, developing and maintaining softwares to writing or translating books, writing and editing in Wikipedia, answering (and asking) in SE, joining an academic club, working on a academic-related project, blogging, etc.

Besides the first two have significant weight in academia, the others seem to be light weight (some even say that they have zero weight) (example for Wikipedia, translating book). Although I understand the worry that I may spread myself too thin, I think the important thing is scheduling time wisely. I don't think they are deserved to be bad effective. In fact some scholarships (like this one) require me to describe "any examples of leadership and involvement in my university or home community". I have even seen some high rep users modestly proudly say that they are active users in SE in their websites. I see a conflict here. Can you explain why contribution to community is deprecated in academia?