Because, gosh, there's nothing worse than musicians who show proficiency their musical instruments. Music should be out of tune, arrhythmic, and full of slowly played wrong notes.
Personally, I like to think of becoming technically proficient on an instrument as a way to expand your musical palette - allowing you to sometimes ‘paint’ sound with more ‘colors’ - when this adds to the music being played.
That said, just like a good painter doesn’t use every color in every painting, you don’t have to use all of your technical skills in every song. To do so can detract from the music.