Due to the fact that they come from "Golem" family - which means they been engineered/ constructed. It same goes for othe Golems suchas: Ice Golem , Cake Golem, Damaged Crystal Golem, Lava Golem (class Contruct). They might look like a "tree/plant", but it is Leaf Golem & Wilting Leaf Golem who are representatives "Tree/plant" as they are "...inhabited by lesser spirits of nature. " (and they are the only golems not in Construct Class). Rotten Golems are mainly connected to rot & decay - which is not very plant friendly. Please see below:
"Though technically actual golems, there seems to be no creator connected to them. Now and then the golems could be observed appearing from seemingly nothingness. They show an intelligent design but seem to lack a specific purpose. The unknown creator, if existent at all, has yet to be identified. Some speculate this creator might be some avatar of rot and decay, somehow connected to the feared pale worm, that is still stuck in its infancy. The rotten golems are seen as the first, mostly purposeless actions of this enigmatic entity. Given their destructive and malignant nature this does not bode well for the nature of its nascent creator. The golems roam certain areas of Zarganash which are ripe with rot and decay, providing an ample and fertile ground for their creation. Their behaviour is aggressive and malignant, yet more sustained by a curious cruelty than any direct malevolence. While only spotted rarely in the past, sightings of rotten golems have become more and more common recently, though their appearance seems to be limited to certain areas that they prefer. A greater spread of the rotten golems is nothing to dismissed though, given that there seems to be an increase of their activity in general.
For now they seem confined to certain areas, where they attack intruders with almost childish excitement. When not facing an enemy they seem to relish in random destruction of environment that also has similarity to a curious playing child. It's not rare that they hurt each other or themselves in such a process. This never leads to infights, however, as the others simply join the destructive play. Despite not actually fighting they still might tear each other apart out of sheer curiosity. All this lead to speculation that the rotten golems might mature over time and gain greater consciousness and intelligence. But in over 200 years from the first reports of rotten golems, such specimens have not been observed."
Source: Tibia Library