One reason for this is, the team member is indispensable, and if he/she leaves, the project cannot proceed till he/she comes back. The lead cannot lead till then. So, they reject the leaves. If the team member has to go at any cost, then they may leave, even if the leave is not approved. In those cases, few leads escalate these issues to higher management.
I did not understand for a long time, why these leads unnecessarily escalate these issues to higher management. The time won't come back. Even if they escalate, next time, when the team member has to go on leave in emergency, they won't stop. The lead would be just spoiling the relationship with the employee. Instead, if the lead praises the team member how important he/she is to the team, and how indispensable he/she is, then the team member would get more enthusiasm, and would try to do something, so that, the project won't fail.
Later, I realized that, these things are used by leads/managers to pass the blame on to the team member. If the project fails, then the lead or manager immediately say that, since this team member went on leave without approval, the project failed. The lead says, he/she cannot deliver the project, when he/she does not know when the team members leave without any notice.
In many service based companies, the team members shuffle frequently. So, even if the relationship is broken, the lead/manager do not care about it.
Of course, if these "so called" leaders get opportunity to work with people like me, everything would be set straight. ;)
If the leader is not ready to take 100% responsibility for the failure, then he/she is not worth of being called as a leader.