Most curricula should probably feature both forms of assessment, demonstrating your knowledge of the basics in a closed book assessment and your ability to produce high quality final products using all resources available to you in take home assignments.
You're thinking of the output as the goal rather than as the means to an end.
Honestly, school essays, as a text genre, suck to read. They are not meant to be enjoyable, beautiful texts! Instead, they are meant to help gauge the student's ability at text planning, the quality of their text project, their grammatical knowledge, and orthography.
I must say I'd be open to seeing schools attempting other models, though, and measuring the results thereafter to see in practice what happens when students type their essays instead. In terms of text projects and planning, that could be interesting and train different skills.