Why insurance companies refuse to give only TPL coverage?

Law doesn't mandate that you should have comphrehensive insurance cover for your vehicle. Only Third Party Liability Insurance is mandatory. But none of the insurance companies allow you to take TPL alone and insist that you take comprehensive cover.

Why? What are your thoughts on it?

It looks obvious that only Third Party Liability Insurance should be mandatory, but any one has reference of any web site about it.

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Probably take full insurance first and then write them to cancel self insurance part from it. They will be forced to cancel it or reply why they can not do it. Next time when you go to take insurance show your last years' policy, that you took only TPL last year so give this time also.

Good thing in such insurance is that you just loose only proportionate amount while canceling your insurance.
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Good idea, but not sure if it is practical. Local branch is likely to refuse that there's any such provision to cancel an insurance cover. You may have to put a strong case and time, effort and money spent will exceed what it would have costed to take full cover...

Some auto drivers do take insurance and then cancel it. No
mechanism for traffic police to verify if the insurance is valid or canceled.
You need not to go insurance office, just send them registered letter. They are supposed to send you money by cheque.