> Then why hasn’t it happened for other macbooks?

It most certainly has, but might not have been publicised. I remember some noise about iPhones screens failing in more or less exactly the same way ~10 years ago. These things are bound to happen regularly, but we don’t hear about it. Ion-exchange strengthened glass is resistant to microcracks, but even so fracture happens.

> The corners of my 2012 are noticeably deformed from impacts and the screen is fine

Glass is an interesting material. It can bend and deform in the right conditions, and shatter in others. Your experience would show that the screen is actually well designed (my 2015 MBP had also a deformed corner after having fallen open from a table; I was actually pleasantly surprised that the hinge survived).