I always felt the fact that there are no mounts as just another reason why the place felt so alien, which is one of the games assets. It kind of adds to the isolated feeling and harsh living of the place. One could imagine good physiological reasons why guars and netches can't be ridden.
Not that I don't use the Guars mod and love it.
I'm pretty sure Guars can be used as mounts. I just checked to see if any of the in-game books say anything about riding guars, and The Poison Song Book VII explicitly mentions someone riding a guar. I'm not sure if the book is just some the Imperial Library site made themselves, but another book called Unnamed Book seems to imply that someone was riding a guar. They don't seem like creatures that couldn't be used as mounts, either.
Now netches are probably creatures that couldn't be used as mounts realistically. They are used for leather and are sort of like cows in that respect, but cows can be used as mounts, netches seem too dangerous and less likely to allow anyone to ride them. Guars have a body that can allow for someone to ride them. The body isn't too wide, which means a guar can be ridden somewhat like a horse.
Now Alts are something that probably don't have the temperament to be trained as a mount.