> You can use other things, like microphone input,

There's now been reported a way to hack a computer by way of its microphone, so that particular method is out.

> or CPU temperature.

CPU temperature doesn't change enough to serve as a secure source of entropy.

> No need to buy expensive hardware generators.

In fact, there are plenty of projects meant to locate sources of entropy to provide more randomness for computer security purposes. The idea of a hardware source like a reverse-biased diode is just one example.

> Then you can use something like Fortuna ...

Interestingly, the Fortuna scheme relies on system sources of entropy other than its own resources to assure security.