The reason could be because typical use cases for Google Keep might involve more single line, quick notes rather than longer ones. A priority is given to quick notes that can be done in a single step, while longer ones require more steps.

I do admit though that on your phone, it's confusing because the icon on the bottom right button is for new line. It appears differently in mine. This raises the age old issue of android apps behaving differently across various OEMs.

Google Keep on Nexus 4