The business of illustration works on the basis that clients commission an illustration then purchase a licence to match the use required. For example, a magazine illustration could be licenced by a publisher for a single country, within one publication, for one month from publication date. The illustrator receives the appropriate fee for this usage licence but retains the copyright.
The reason illustrators retain copyright is simple: we have to make a living. If we lose copyright to our work we have no control over how it is reproduced, altered, or even sold to third parties. We would lose the right reproduce our own work and payment for any secondary uses. We could even find ourselves competing against our own work.