It's not a simple answer, because its not an answer to any question. It is "simpler" to assume the thermometer readings exist without any thing to cause them, then they move up and down, and its "simpler" to assert: it is the nature of thermometers to move up and down.
This kind of simplicity isn't simplicity at all, it is to abandon saying anything. It is in the nature of consciousness to inexplicably have all the properties which are needed so as to seem the way it does, sure.
And what is this "consiousness" in the end, which has in its nature, the production of all material reality to serve as a fixed causal basis for perception? to generate perceptions of the brain and bodies of animals; of our death, and of a world which is describabl without any of its own properties? What is the nature of a consciousness which deliveries to us a world that requires none of it?
What is it that when I move the muscles of my eye, and what i see changes? What is it that i require a light in a room to see at all? That i require it to be the case that whatever I perceive, my fixed perceptions must always be as-if the laws of physics were true? What is it to say, "consiousness has the property of seeming as if when I see, I see because light scatters off a surface into my eye, and I can control the image genrated by moving the muscles of my eye?" and yet all of that sentence be false without the word 'seeming' ?
What madness to is it to say that mercury in a thermometer not only acts as-if it is in coffe, but in its nature, acts as if there is an entire world that it is moving through -- and its motion up and down is always according to laws and principles as if such a world existed?
This is no longer consciousness at all. When the nature of mercury in a thermometer is to act as if coffee exists, it is no longer mercury. When "consciousness" is taken to have all the features needed to provide the material world, it is no longer anything in a mind -- but has within it, the whole of the external, fixed, material -- and so it is itself now alike those things. When "consciousness" has finally been modified to produce everything within it, the term means nothing at all.
The entire system of properties and objects which are external to consciousness, narrowly defined, are still external within consciousness broadly defined. And with this broad definition comes all the laws of physics, all the properties of materiality, all of everything which mentions nothing of sensation. And the word "consciousness" has to bare all these properties? And you call this simple?
It is much simpler to answer the question: why does the mercurary in the thermometer move? is it because in its own nature is a simulation of an entire universe? No, its nature is simple. It is that there is such a universe it is inside.
And our consciousness is likewise simple.