Q. Why are all resolutions in ratio of 4:3?

The bad news is that almost every monitor can only display upto a maximum of 262,144 colours (i.e.  18 bits/pixel Colour Depth). The other bad news is that human eye can only observe a few million colours at the most. So even if you had lots of bits per pixel as well as very advanced display systems it would be useless.  Perhaps this is good news rather than bad news for hardware developers.

This also implies 24-bit colour bit-depth is the practical upper limit. Therefore this depth is also known as true colour since with this depth system stores more colours than can ever be seen by human eye and therefore it is a true colour representation of image. However 24-bit colour or true colour systems have more colour than possibly useful they are suitable for designers since they assign 1 byte of storage for each of the 3 additive primary colours (red, green and blue). A number of new systems even have 32 bits per pixel.  Why?  Essentially the additional bits aren't used to hold colours however something known as an Alpha Channel. This 8-bit Alpha Channel stores special effect information for image.