Because you don’t know how to set up your system correctly. 
In fact, it’s not your fault. Such things should be set by settings. But you can change this manually. Create a file in /etc/phosh/ called phoc.ini and enter these lines:
[output:DSI-1]
scale = 1.5
or instead 1.5 (150%) you can choose any other scale you want. After each restart it will be set to this value. And written in etc makes changes persist against updates. It overwrites the lines of /usr/share/phosh/phoc.ini, which can be updated and so they’re not persist.