Is It Possible To Build A Laptop From Scratch?  

I think it can't be done by just simply buying the parts...Besides motherboards are different and so with the laptop casing. Anyone who can assemble can teach me.

The answer is... sort of. You can buy the laptop shell which usually has the motherboard pre-attached. You can then add a notebook hard drive, RAM, DVD drive, etc. Unfortunately, it is extremely difficult to get the laptop shell and motherboard on its own. In addition, it is not cheaper than buying a branded laptop. Building a laptop with the cheapest crappiest components will cost over $700. You can buy a medium quality branded laptop for $599 if you know where to shop.

As to your first statement, it is not a lot cheaper to build your own PC. Sometimes it's cheaper, sometimes it's not. It is cheaper to buy a branded PC if you are looking for a cheap machine. Branded budget PCs are $50 to $100 cheaper. For gaming PCs you can save anywhere from $100 to $1000 if you build it yourself.

It's possible but it not common so the prices are higher and the pieces more difficult to find. In the end you could pay more to build yourself.