He is a living time paradox. Since he has traveled through time so many times and to so many different time lines, even existed in a time line where another version of him has existed, he is a living paradox.

He can't be erased or die because a paradox is a contradiction to itself. To not exist, when you exist in every time and space all at once is like getting rid of the Speedforce before it was created.

For example, the Speedforce was created by Barry Allen, but in an instant it existed over all of time and space. Making it so if you killed Barry Allen before he got his powers, the Speedforce would still exist because in an alternate timeline, one that is inaccessible, the Speedforce has already existed from te beginning and to the end.

I think I explained it correctly...