Why people don't have your back
The issue with people is not that they don't have people in their corner. It's that they ignore the ones who care and invest all of their time and energy into the ones who don't.
Think about it from their perspective. If you were doing everything in your power to make someone happy and improve their life, but they ignore you, never have time for you, etc. Then their actions are stating that they don't value you. Now when you look over to see someone who doesn't give 2 fucks about them getting all their energy then you are at this point being disrespected by them. No one wants to be treated like shit, so eventually you stop investing in this person and move on with your life.
This is exactly what is happening to the people who used to be there for people who complain no one is there for them, so instead of blaming other people you need to stop investing in the people who don't care and start investing in the ones who do.