Basically, making anyone who contributes aware that the contribution doesn't give them a claim in the copyright of the project. Short and simple.
It's the social issue, not the legal issue, that's annoying about CLAs.
It is as simple - and it is not. You need to take care of special cases, like contributions from company employees in their free time (their company could still own rights to this work), people contributing other people's code (SO answers), patents and whatnot. Fortunately there are existing CLA agreements (Apache for instance).
As for this being the social issue, I don't know yet how big a problem that is. It's never bothered me before, as I recognize that maintainer might want to take the project in another direction in the future and since I don't want to maintain it, I am just happy that they are doing it for me. I will use CLA for my project and if someone doesn't like it, it's fine too - I don't mind forks (if they are well maintained, I'll just switch to them ;), and if someone doesn't want to contribute because of this, I don't want their contribution to be in my code anyway, because it limits my options in the future. There's a new post today on HN [0] that presents options pretty nicely, and it is very aligned with the conclusions I came to.
[0] https://www.influxdata.com/blog/its-time-for-the-open-source...
Maybe because then you need to have CLA's (Contributor License Agreements)? Otherwise, who is "you" that they need to ask? It's not just your code, it is based on work of many other contributers.