Why we can't pass Object either Standard or Custom in Future method as a Parameter in salesforce?

Hello Suraj

Reason for you can't pass Object either Standard or Custom in Future method as a Parameter

The reason why sObjects can’t be passed as arguments to future methods is because the sObject might change between the time you call the method and the time it executes. In this case, the future method will get the old sObject values and might overwrite them. To work with sObjects that already exist in the database, pass the sObject ID instead (or collection of IDs) and use the ID to perform a query for the most up-to-date record.
And also There is no direct way to pass SObject record as a parameter in @future method, but you can make one hack where first you will serialize object state in string and pass this string to @future method and de-serialize  it again to convert it back in SObject.

For Example

trigger MyTrigger on Test_Object__c (after insert) {
String objJSONStr = Json.serialize(Trigger.new[0]);

MyFutureClass.myFutureMethod(objJSONStr);
}
public class MyFutureClass {
@future
public static void generateEtsPdfAttachment(String objJson){
Test_Object__c obj = (Test_Object__c) Json.deserialize(objJson, System.Type.forName('Test_Object__c'));
}
}

For more information you can lookup here:https://developer.salesforce.com/blogs/developer-relations/2013/06/passing-objects-to-future-annotated-methods.html

Hope it may help you: