I read that Apex constructor runs when the VF is being generated, but it doesn't not in my case

So i have this Apex code and VF code.

public with sharing class ItemsToApproveA {


private final string UserID = UserInfo.getUserId();
private List<ProcessInstance> ProcessInstanceID  {get; set;}
public String getUserID() {return UserID;}

public void ItemsToApproveA(){
  ProcessInstanceID = [SELECT TargetObjectId 
                            FROM ProcessInstance 
                                Where Id IN (SELECT ProcessInstanceId 
                                                FROM ProcessInstanceWorkitem 
                                                    Where ActorId = :UserID)];   


}

public List<ProcessInstance> getProcessInstance() {

    return ProcessInstanceID;
}
                                                 
}

<apex:page controller="ItemsToApproveA"> 
   <apex:pageBlock title="Items To Approve">
      <apex:pageBlockTable value="{! ProcessInstance}" var="Items">
         <apex:column value="{! Items.TargetObjectId}"/>
      </apex:pageBlockTable>
   </apex:pageBlock>
</apex:page>

The ProcessInstanceID variable doesn't get assign with any values, unless I move the code below into the getProcessInstance funcation :

  ProcessInstanceID = [SELECT TargetObjectId 
                            FROM ProcessInstance 
                                Where Id IN (SELECT ProcessInstanceId 
                                                FROM ProcessInstanceWorkitem 
                                                    Where ActorId = :UserID)];