Issue with After Trigger And Before Trigger in Salesforce

Hi All,

this trigger is not working

trigger ClosedOpportunityTriggera on Opportunity (before insert,before update) {
List<Task> toBeAddedTasks = new List<Task>();
for(Opportunity A:[SELECT id FROM Opportunity WHERE StageName = 'Closed Won' AND id IN :Trigger.New])
{ toBeAddedTasks.add(new Task(Subject = 'Follow Up Test Task', WhatId = A.Id)); }
upsert toBeAddedTasks; }

it is working when i change before insert to after and before update to after

trigger ClosedOpportunityTriggera on Opportunity (after insert,after update) {
List<Task> toBeAddedTasks = new List<Task>();
for(Opportunity A:[SELECT id FROM Opportunity WHERE StageName = 'Closed Won' AND id IN :Trigger.New])
{ toBeAddedTasks.add(new Task(Subject = 'Follow Up Test Task', WhatId = A.Id)); }
upsert toBeAddedTasks; }

i need to know why it is not working similar in both scenario

thanks