I often see people saying that the linux kernel does not have a concept of proper power management. Can somebody explain what is usually meant by that? Also why is this the case and why the developers have not been investing more resources in this area having in mind that there are so many laptops these days and power management is a growing issue?

I think that Linux is mainly used for servers and/or that's what the major companies are interested in, therefore the main focus for power mgmt is mainly what is relevant for that market segment (e.g. disks + CPU, to achieve maximum throughput).

Having said that, I personally that it's a miracle that e.g. PSR (panel-self-refresh) for Intel GPUs is being implemented for Linux, but after having seen that it does not work properly on any of my 3 laptops (Asus G771J, Dell XPS 13, Lenovo X1 carbon), I think that very few resources are allocated at this specific example, basically reconfirming my above statement... .

The Archj wiki article might be a good place to start your research.

https://wiki.archlinux.org/index.php/Power_management

^ which would indicate that the kernel DOES have some concept of power management.