Because they are absorbed in the endless struggle to think well of themselves.

From The Cocktail Party, 1949 by T.S. Eliot

Half the harm that is done in this world
Is due to people who want to feel important.
They don't mean to do harm — but the harm does not interest them.
Or they do not see it, or they justify it
Because they are absorbed in the endless struggle
To think well of themselves.

That's the one I went looking for but there are others from the same source.

You will change your mind, but you are not free.
Your moment of freedom was yesterday.
You made a decision. You set in motion
Forces in your life and in the lives of others
Which cannot be reversed.

and

The destination cannot be described;
You will know very little until you get there;
You will journey blind.

and

We must always take risks. That is our destiny.

and finally, the cruelest truth about good intentions encountering reality.

If we all were judged according to the consequences
Of all our words and deeds, beyond the intention
And beyond our limited understanding
Of ourselves and others, we should all be condemned.