Why do people ghost?

I've friends who were pretty close to me and now are residents who have bailed out on helping me. They give all sorts of excuses.

I've had some contacts who have completely ghosted me.

Why do people do this? They could just email the PD of their program, it's not like they're losing anything.

Fortunately there have been very rare nice people too who helped me.

One attending that I knew has blocked me on WhatsApp when I asked him to help with recommendations. How mean is that!! I feel very insulted and hurt.