However, I read somewhere that humans are not good sources of entropy.... so maybe that is not a good idea. We problably keep moving the mouse in some crazy pattern.
I always assumed this isn't the only source of entropy they use, but it's added to some other random generator in the computer. I've just tested it on
https://www.bitaddress.org: without using my mouse (Tab many times to be able to type in the entry field), I filled it by holding the A-button. After that, I did the same again. As expected, this produced 2 different private keys.
I believe the idea is to address the risk of having flawed RNG because of the computer being compromised and not producing truly random numbers when directed.
A compromised computer producing not truly random numbers is unlikely to produce a collusion after two 'random' events. They will rather produce random numbers in a smaller space. The output will appear random without testing, but someone with knowledge of the specific space numbers will be generated will be able to generate a collusion with fairly low effort.
The movement of the mouse is intended to counter the above risk in adding user specific random to create a larger space of possible private keys even if the computer's random function is compromised.