Carbon in the form of carbon dioxide as raw material is literally instantly available and the resulting "composite" is lightweight. This is not to say nature always considers recycling beforehand. Nature needed to invent fungi to break up this carbon dump. Carbon era is named thanks to this fact. And it's tempting to think nature "came up" with humankind to make the humongous carbon dump of the carbon era useful once again.
Really, you have causation backward. Eg. the supposed Carboniferous Period was an opportunity for something to evolve which consumes wood. Many "problems" get solved by accident in this way, but many also don't - as long as they take place over a long enough timespan. If they happen too quickly, there's not enough time for living things to adapt.
Currently there is an opportunity for an industrious plastic-eating microbe to hitch a ride in every gut on the planet, deciding the winners and losers of the plastiferous period. All that means, though, is that there's a chance such a creature could appear and take advantage, not that it will happen. (Yes I know plastic-eaters have been discovered, but I'm not aware of any having an effect on the fitness of other creatures.)
>but it would be interesting to ponder how they would have evolved on a metal planet
It overlaps a whole lot with the concept of a dyson trees, but the core problem is that it needs to be able to use the metal in the first place - earth is a metal planet, in the sense that ~10% of the planet is iron, and yet our trees are not steel.
I don't really think that's a fair assessment, since most of it is in the core. That's some very long and heat resistant roots.
I also can't help but wonder, could trees even use iron if it was plentiful in the upper crust? You need a lot of energy to separate iron oxide into elemental iron. Betting against what evolution can make is usually a bad idea, but that would be a neat trick.
