Why do most projects do not 'close' correctly?

The one and only reason for this is project mismanagement at Project Execution and Control level, especially when multiple department heads are involved and each stake claim to process ownership, there are subsequent changes to project plans.
The best way to go about is to identify and involve all stake holders and process owners before the final project plan is drafted, give them time to study, recommend changes and approve the final process map that will then become a part of the project plan.

I found that project closure is rarely included in the project plans.

If you mean that project delays have been creeping during execution, I suggest that is a failure on the project manager's part. He/she should identify any deviations as soon as they start and decide on how to fix the problem and seek management and customer approval if the fix is going to cause any change in the project goals, cost, or time line.

There are many factors might affect on the project:1.
Improper planning for the project2.
Weak top management support3.
Un-qualified Project Manager4.
Insufficient allocated time or resources including team members allocated time for the project5.
Politics inside the company

This happens due to lack of consideration of proper project closer strategy, which can prevent from all delaying factors and can make team stick with there objectives towards that project.

Yes , from your1st day you have to identify yourscope & stake holders ,, give them enough time to study collect  all the recommendations and you have to tak approval on your scope and deliverables in certain time and cost within the proper quality required

Improper closing of the project because we don't specify all deliverables from the begining (before execution). Deliverables could be any thing such as hardware, software, licenses, services, etc.

PM is responsible to make a list of all deliverables will be handed over to the customer so he can get the final approval/sign off.

 

 

It happen before the end of execution and is responsibility of HR.
In both case either it is success or failure team members become overconfident or depressed receptively and forget the requirement of close out of project as per terms and condition or regulatory system.

Thanks for the question, as you mentioned Rajinder, scope creep is one of the reasons of not being able to close project properly, scope creep is usually part of people craving to have some improvements/changes to what they currently have, which sometimes reflects on the project requirement definition, that may not be well defined after all.
Another reason is our cognitive ability to actually know the detailed inputs and requirements for all activities within the scope, which would lead to halt an activity, and create another activity (or project) to be able to complete.
Also, as we progress in the project, we learn many things about the company that we need to do, sometimes a project manager elects to add them to the project, sometimes are just recorded in the lessons learnt report.
As a conclusion, we do not close project completely because we do not know exactly what we needed, we do not really understand the company, and did not consult with the right people when initiating this project (and this is not necessarily the fault of the project manager, it mainly depends on organisation maturity in projects).

There could be many reasons based on the environment, team involved, type of project etc, however to look at it from project management perspective the key things to make sure projects gets closed are 1. Defining Success criteria of the project2. Having RTM(Requirement tracebaility matrix) and importantly3. Having a lifecycle and defining phases and gates and deliverables expected from each of them. As a PM it's important to verify the scope and agree on every milestone and get the required sign of.

Projects do not close correctly mainly due to variations on order (Scope Creep) a large percentage of the time this occurs due to the lowest bidder being selected for the tender. 
Then all the VO's creep in because the lowest bidder missed aspects of the project invariably because of insufficient planning pre-project.
Getting payment approval on a VO is like getting blood from a stone.
There is always a reason why the lowest tender is so low, either inferior product or the entire scope of the project was not allowed for by the applicant.