No four words can wreck a friendsjhip QUITE like "why don't you just..."
It is the gateway to judgey, unsolicited advice and the quickest way to let someone know you don't understand their situation. While you may think you are dispensing sage wisdom, the receiver ain't receiving. I knoww that when people start a statement like that, I instantly think: "Why don't YOU just shut your face!" Often times people just want to vent so the unsolicited advice isn't even necessary. And the term will likely do more harm than good.
If you feel there is something that just MUST be said, why don't you just rephrase it and save yourself a cold shoulder, a severe side eye in perpetuity or being sent straight to VM forever and ever AMEN.
Many times we fail to see that we are "at fault" not in what we say (because if we are all being honest, certain things must be said [I loves me a good paranthetical phrase, don't I?]) but in how and WHAT we say. Maintaining a healthy friendship is about finessin that thing and not coming from a place of harm.