Google's Material Design documents only mentions following:
Touching a text field places the cursor and automatically displays the keyboard.
Text fields - Components - Google Design Guidelines
Same is said in API Guide for Android developers. Neither mentions the reason why to show keyboard only after the user have tapped the text field.
Going on a hunch, I think has something to do with limited screen estate. You wouldn't want to block the user from seeing if there something to see on the lower portion of the screen.
And to why some apps show the keyboard and others don't, I don't know, especially if they are alike. It might be that designers of another app think there is something worth to see at a glance and the others that there isn't based on their intuition and/or user testing.