Sometimes people in leadership roles fall into a trap of wanting to be liked, and don't hold people below them accountable for their actions and the results they achieve (or fail to achieve). Or, a busy leader may feel that holding people accountable is time consuming and tend to procrastinate, or avoid accountability, particularly if the leader is prone to avoiding confrontation.
Apart from the obvious issue that leadership silence in the face of poor behavior tends to condone that poor behavior in the mind of the person exhibiting it, the real downside here is what other followers or employees will perceive.
Followers expect (even demand) that leaders take action when required. They look to the leader to help them, and to ensure that everyone abides by the same rules. When a leader fails to hold someone accountable, it sends the message that the leader is not serious, resulting in a form of bad behavior contagion. It spreads, because employees take their cues from how the leader behaves, rather than what the leader says.
You want people to take responsibility? Make them accountable. Failing to do so while make you a non-credible, and untrustworthy appearing person who is not worthy of loyalty and trust.
It's understandable that there's tendency to allow "little things" to slide. But it's often the little things that set the organizational tone.
Have high expectations of the people around you, and show that the work people do is important enough to hold them accountable.