This is only from little experience with Ubuntu and the like, but it seems that mainstream Linux distros, by their very nature, contain many separate components installed by default that could be doing something with the CPU despite not really being necessary, with no one group responsible for overall power management/resource use (unlike Windows or MacOS).

It may be open-source and free, but if the user doesn't want or need it and it's installed and actively consuming resources, it's bloatware nonetheless. I'd be interested in knowing the battery life with a far more minimalist distro.