Because an alternative implies that the original modality is a viable one, and that the alternative simply offers a different route to the same destination. This is not true when considering the animal model, because it is not a viable modality.
Many use the word alternative to mean any test that does not harm animals. The problem with this is that it validates using animals as predictive models for humans. Looking for alternatives to tests that don’t work in the first place has been primarily a subterfuge for more funding for animal-based studies.
So how does one properly define the word alternative?
The word alternative comes from the Latin alternare—meaning to interchange. According to The New Oxford American Dictionary it means: “One of two or more available possibilities.” The important thing here is that it implies viability.
A scientifically invalid practice cannot be replaced with an alternative. Consider this example: Eating broccoli is not an alternative to eating rocks for nutrition, but it is an alternative to eating asparagus.
The Encarta Dictionary defines alternative as follows:
Other possibility: something different from, and able to serve as a substitute for, something else.
Example: You could take the bus as an alternative to driving. Note that the original choice in this example—taking the bus—is viable.
Possibility of choosing: the possibility of choosing between two different things or courses of action.
Example: We gave you the alternative; you decided to stay. Again, the original choice—leaving, presumably—is the original and viable choice.
Option: either one of two or one of several things or courses of action to choose between.
Example: I can’t decide which of the two alternatives is worse. Both are viable, just not great.
The Cambridge Dictionary defines alternative as follows: “Something that is different from something else, especially from what is usual, and offering the possibility of choice: an alternative to coffee.” The original choice is viable—in this case, coffee.
Another example: There must be an alternative to people sleeping on the streets. The original is viable; in this case, people are actually sleeping on the streets.
I’m afraid I have no alternative but to ask you to leave (that is what I have to do). Again, the original is viable; in this case, staying is viable if the person behaved better.
The opposition parties have so far failed to set out an alternative strategy. The original is viable; in this case, the original strategy is viable, just not acceptable to all.
An alternative venue for the concert is being sought. The original is viable; in this case, the concert was scheduled for a certain venue and could have been held there, but now needs to be changed.
We could go to the Indian restaurant or, alternatively, we could try Italian. Again, the original choice is viable because indeed the Indian restaurant serves food.