There is difference between metrics and KPIs. KPIs are focused on measuring performance but metrics are more general. Any measurement can be a metric. For example number of people in different phase of a project is a metric but any measurement on reducing cycle time is a KPI since it is directly relevant to performance.
Now that you got the difference, let me explain you simple the difference about routine jobs and projects.
Routine jobs obviously are a part of team or individual job that should be done regularly. For example generating electricity for a plant. To focus on such an activity, we need certain KPIs to measure the performance and improve it but regarding a project we have start point and end point. The job is not considered as a routine job at all. Actually when the project is delivered there is nothing to do after that. (In some cases maintenance and keeping the quality level can be considered as a routine job after project delivery!)
By the above explanation, I can speak about the reasons:
Many projects are implemented for the first time. In these cases, we don't have any history to set the targets based on that. Therefore, when we would like to study our control plan we need to define some other metrics to prepare enough data to analyze the problems and the cause of missing targets. This is not a method for excusing ourselves but to understand the situation ;-)
Chance of improvement in projects is very limited. It is not similar to routine jobs. We implement the project once! It's not easy to find routine activities when you are doing a project. This means that we need some supportive measurements to monitor project KPIs closely. For example: The cost of materials in a project is very important and it is a KPI but we can have some metrics such as inventory turn-over as a backup data to warn us on daily operation and the usage.
Since we have limited time to finish the project, reporting system for higher management should be simple and informative. These metrics are great guidelines for project managers to decide better.