Why do movie companies do this?

2 or 3 years is the average time for completing a good or perfect movie. there's lots of things to do in the process of film making. i am interested in this field and want to make carrier in film industry.

The director has nothing to do with it and probably feels the same way you do. It is the producers and movie studio that decide when to release a film. Sometimes it is because of the movies they already have sleighted to be released (i.e. they don't have the budget at the time so they push it back) Other times it's just not what they're really looking for at the time and they pull it out at a later date when it's more appropriate to the social attitude.

And sometimes it's for political reasons within the studio that you will never know about--contract issues, copyright issues, etc.

That's how long it takes to complete a lot of movies. Especially big pictures, big blockbusters or expensive films with a lot of effects and the like. Doesn't happen overnight.