Is there any disadvantage of buying a refurbished iPhone 6s/7? I found some websites which sell "Refurbished" iPhone 6/6s/7 . Refurbished iPhones claim to work like "new". Is it really true? 

Some people told me that - the battery in refurbished iPhone is Not new. Is it true?

Most of the websites give 60/90 days warranty for refurbished iPhones. Why don't they give 1 year warranty? Does it mean that refurbished iPhones don't last long because the phone was already used by someone else (which causes the phone to lose some of it's lifetime) ?

If there is NO disadvantage of a Refurbished iPhone (for eg: I will get 1 year warranty with Refurbished iPhone) , then what is the point of buying a Brand new iPhone spending extra money?
It depends where you buy the iPhone. Generally it’s wrote when the battery is new.

On the Apple’s refurb, the batteries are new.

For the warranty, the websites offers 60/90 days because it’s not NEW iPhone, not because they’ll be decreasing shortly.
And the websites would tells you the level of the iPhone, if it gets scratch on the back, missing wire.