Like @Furdiburd explained. it is not really a choice made by Fairphone. Quickstep is the launcher made by Google. and Google is Google so they wanted it this way…
if you don’t like it, its a option to install a launcher you prefer.
you can check this wiki page for different launchers how are already tested by Fairphone users.
and feel free to try a different one how is not added to the wiki yet.