First, you don't define "value", so I'll give the definition I'll be using for this answer :
"A factor attributed to a being, an object or a service used, when faced with a choice between several options, to determine which one will be favoured."
Now, from my understanding, the question "Why do ethics always consider that lives are valuable ?" (correct me if I didn't understand the question) stems from the question "How does the nature of Humans make them define ethics that always value lives ?".
Notice that I don't keep the "why" in the question because it implies that there would be some kind of "purpose", an arbitrary "reason" for this. Which could only come from a dogma that defines an "objective" sense of "value" that would depend on which dogma the individual adheres to. Making the question entirely opinion-based.
My short answer is : Life's most basic mechanism
Life develops by self-replicating and altering its environment to make it more favourable for the next generation. This induces inter-dependencies that lead to the necessity for life to attribute more value to the living than for the non-living. As a living being that would attribute not enough value would be likely to destroy its living environment, on which its survival and that of his offspring depends. Leading eventually to extinction through basic natural selection process.
Now for the details...
It's simple to understand why someone will always attribute value to his own life : a species that commits suicide every time it faces a "choice" cannot survive.
As I already said, living organisms (especially complex ones like Humans) rely on others to survive and develop. So, as a basic consequence of natural selection, species that didn't attribute value to what is beneficial to them disappeared. This effect is further increased through Evolution, as it is a huge evolutional advantage.
As a species that takes care of its offspring, Humans needed to develop empathy to help project the value attributed to the self onto their children. Species that didn't take care of their offspring (like some fishes, reptiles, etc), often hid or buried their eggs to lessen the chances they had to find them again later and eat them.
As social animal, we Humans also developed an even stronger sense of empathy that extended to many individuals.
It also extended to domesticated animals (that quickly became a key element to Human development) though it is not clear if it needed any evolution process or if it's just that empathy applies to anything that has a behaviour that resembles our own (you can have empathy for a suffering ant as you can recognize the way they twist their body as pain, you can have empathy for drawings, etc.) which ended up being convenient to tame/breed animals.
Note that, in the same way, we don't know if empathy existed before it was needed to maintain viable relationships as social animals, as it could have been a tool to better understand preys for example. But when it happened is not really relevant to us anyway. But in any case, that could mean that empathy may not necessarily involve attributing more value to the lives of others.
I think this answers the question "How does the nature of Humans make them define ethics that always value lives ?", but now I'll try to answer your original question :
In larger Human groups where a defined society appeared. The relationships between people are a lot more complex, and can be impossible to handle for individuals. Two new concepts appear in the equation : mentalities and dogmas.
- Mentalities are an organic result of the processes I mentioned earlier (empathy, environmentalism, self-value, ...) but are very hard to define given that every individual of this large group has a different understanding of these depending on his own personal experience.
- Dogmas are attempts by society to simplify what the mentalities aspire for by defining arbitrary rules that are supposed to establish "objective values". Dogmas make handling interpersonal relationships a lot easier.
Both dogmas and mentalities, as they ultimately come from the same processes that attribute value to lives, have a tendency to keep that principle intact.
However, society factors are a double edged sword when it comes to attributing value to lives :
- Mentalities are not shared by the whole species (unlike empathy, self-value etc.), they are shared only by the group that defines and follows their principles. And the differences between the in-group and the outsiders induces many fallacies that often lead to attributing more value to the lives in the in-group, and less value for the outsiders.
- Dogmas are monolithic, and have a hard time adapting to the ever-changing mentalities. As a result of the previous point, a rule reducing the value attributed to the lives of outsiders could be set in stone by a dogma. Then, mentalities evolving could lead to people wanting to attribute more value to outsiders, but the dogma will still dictate otherwise.
In my understanding, Ethics are the take of one individual (or maybe a small group) on the mentalities and/or dogmas, and how those should be according to him.
As Ethics come mostly from an individual's perspective, they are more likely to reflect the attribution of value that comes from individuals feelings like empathy, than group biases. Which could explain why Ethics almost always attribute more value to lives, even when they come from a society that doesn't do as much.
In general, Ethics tend to define a higher moral standard than what society would define because it aims to satisfy the ideals of every individual even including outsiders (in general) whereas mentalities and dogmas just aim to satisfy the average of the group.
Which leads to Ethics almost always attributing more value to lives.