by default in /var and not in /usr/var/ ?

[linuxcbon linuxcbon]

On Fri, Oct 23, 2015 at 1:42 PM, Simon McVittie
<simon.mcvittie at collabora.co.uk> wrote:
> On 23/10/15 13:29, linuxcbon linuxcbon wrote:
>> Is it possible to have them in /var instead by default ?
>
> Not without breaking the ability to use --prefix=/opt/my-app-bundle for
> people who want to do that.

But in which config files is it written that it is in /usr/var/
instead of /var ?
Why not change them, and still allow options like prefix, localstatedir etc ?
How do other softwares manage it to have different default folders and
still allow /opt/ etc ?

Thanks,
linuxcbon