As someone who's done work fixing other people's work, concepting and designing, and also doing nitty gritty asset creation pipeline stuff, concept art is often the most boring and least useful part of the entire workflow unless you have done any of those other things already.

If you're making character concepts, for instance, you need to be able to not only show it from different angles, but also show how the character moves, facial expressions, silhouette and shape language control from distances, and a ton of other things that matter for the production staff.
These are all things that take awhile to grasp even when you're working in making the assets, let alone simply designing and doing concept sheets.

Imagine then that you have to do this for way, way more than the exciting stuff like characters. you have to think about how many, many different objects, scenes, vehicles, animals, etc go into something as well. If you're not like me where I just love being able to build all that stuff out and you only care about your blorbos, you are simply not getting very far.