Why does antenna length have to be at wavelength
Only a full wave antenne length will create an optimal radiation pattern. You can play with the antenne length by adding capacitors or inductors but at the same time you do no longer create an optimal radiation pattern. 

In many situations you have to use capacitors and/ or inductors to reach the goals you have set. For instance if you want to create a multi band antenna. This kind of antenna is in frequent use with radio amateurs.

The picture shows you a multi band antenna where resonance has been reached by introducing tuning elements.