Samsung phone defects

I have a Samsung J7 pro for less that 6 month. One morning when I woke up I stared noticing my screen starting to go go black starting from lower right hand corner. This increased and I was unable to see anything on screen. Took it in to Samsung somerset west who then send it off to WeFix in Canal Walk. A day after received an email with quote of R3000 to repair screen. When I called them they said they found a fine hairline crack on the inside of LCD screen. How can this have appeared. Firstly i would expect a bigger crack if phone fell. Secondly, I bought all this protective covers and screen protection that should prevent phone damage in the event of being dropped. With this, could this not be a manufacturing default. Im am seriously disappointing in Samsung!!