Do you expect applications not to use CPU? How do you think computers work? Yes, even CPU monitors need to use CPU time to work.

Perhaps your question is, why don't applications use CPU more efficiently?
Well as far as I can tell, most of the applications in the extras repository seem to be quite CPU efficient. If you're installing outside of extras, then you're installing beta software which probably isn't fully debugged.
And then there are bugs with the OS software itself that can cause high CPU usage in conjunction with other applications.

The answer is, of course, not to install any apps if it worries you too much. Alternatively, learn to program and debug the code.

Realistically, the point of having a device like this is to use the apps, so you shouldn't worry too much if it uses CPU - only worry if it uses so much CPU that you can't do anything else on your phone.