Well, there can be. Windows machines often don't have ECC RAM, but they can.
It's not so much Windows versus FreeNAS hardware as the goals are different. A Windows gaming machine is usually not well-suited to be used as a server. FreeNAS systems are usually put together with different goals, not for speed but for reliability and resources. A closer fit would be hardware meant for a Windows fileserver.
just because something *can* does not mean it *should*
zfs needs at least basically 4gb of ram to operate. 8gb is the FreeNAS listed *minimum*. windows will run fine with 4-8gb, and barely notice anything about that, unless you are running VM's, or some kind of graphics/video editing, or like 4 games heavy games at once.
FreeNAS will run better the more ram it has, more caching = instant speed as less hard drive access is required: more RAM means a cache drive could be useful (need a lot for a cache drive to even be usable at all)
a windows fileserver just won't have the same RAM requirements, because it doesnt use it the same way that zfs and linux/unix/etc do. unix based systems keep RAM FULL of something most of the time as a cache, dropping cached info on the fly for programs that request it and recaching when/iof it become available again. windows just does...nothing with RAM over what is actively in use (unless you have a RAM leak program, then it uses everything up till it stops entirely).
ECC does pretty much nothing for performance, its only purpose is data integrity.