Just because the protagonist has been reincarnated doesn't mean he suddenly became better, especially if his entire previous life was just suffering.
Improving is a process that takes time, introspection, courage, and willpower—things that Saito never had before, so everything has to be built from the beginning.
As for the events that occur, Saito has always let himself be carried by the situation and has never been an actor or made decisions. With his reincarnation, he finally starts to act for himself and others.