The reason that there are so many standing committees in the two houses of Congress is that the federal government makes laws that cover such a huge variety of topics. Because it does this, it needs to have a lot of committees.
Committees allow the real work on bills to be done by people who can be relatively expert on a given topic. This means that not everyone in Congress has to try to educate themselves on every bill (they'd never have the time to do that). Committees, then, are very important because they let Congress get more work done on more topics.
If Congress considered fewer laws on fewer policy areas, fewer committees would be needed. As it is, there are many committees to cover the many areas of public policy that are legislated.