Very large telescopes are not very useful to use visually. The main reason is that in order to take advantage of all that light gathering power in such a large telescope, you need a lot of magnification, and Earth's atmosphere makes using large magnifications pretty much useless. Using a more normal magnification on such large scopes wastes a lot of light and it will do little good.

A camera sensor or film plate doesn't have this problem, because light doesn't need to be returned to a collimated form for the camera to use it (like our eyes require). Additionally, cameras can take long exposures, gathering far more light than a human eye can, therefore allowing us to capture very faint light out there. Generally speaking, the human eye is not a very useful instrument for detecting things deep in space… at least not until a camera images it and presents the image to us.

Because of this, there is little benefit to direct observation when it comes to professional astronomical work. Therefore professional astronomers don't really spend time looking through telescopes as part of their job. Visual astronomy is really only good for the asthetic value of seeing some of the great wonders of the sky with one's own eyes.

A camera can take long exposures which shows more detail, this is the main reason. A camera can also be made to see in wavelengths that the eye can't see.

Because they can collect and process more data using the pictures and electronics than they can by looking through the telescope. It is fun to look through a telescope and see things directly, but much more useful to take pictures, gather data, etc.