There's why. The content and website layout is adaptable to the smartphone/tablet size. To make it easier to understand we can make a little experiment. Download on your device Google Chrome browser and in the right-up corner you will find three dots. Click on them and then on "Request a desktop site". In this case you can see how would appear if the site doesn't change size and layouts. Everything is very small, the information is gathered together and if you are not careful you can click without wanting on something else. So, it's more easier how it is now. You can also disable this feature to your website if you want.
because of different platforms, mobile is another platform and PC is another platform that y these websites are different in nature, and some times these mobile sites are much less heaver then the pc website respectively