で in this sentence doesn’t indicate a location where some action takes place. Its function is to delimit the scope of comparison. It is used in a sentence that expresses the comparative (“more”, “better”, etc.) or superlative (“most”, “best”, etc.) degree, such as this one, which contains the superlative adverb もっとも.
Your other example, with 一番, also fits in this category.
In these particular examples, what comes before で just happens to be a place but it doesn’t have to be, as you can see in the following, rephrased sentence.
で here limits the scope of comparison to (all) land animals on Earth.
The scope can be much narrower, as in the following sentence with a comparative.
Here the comparison is limited to between the lion and the cheetah.
In the following sentence, で does indicate a location where some action (生きる) takes place (except it doesn’t, because the sentence is negative).
に doesn’t work in your sentence because it should be used to indicate a location where something exists or is present.
Note that the following sentences, with the verb ある, both talk about the existence of something.
And the following talks about the absence of something.
Your first sentence, on the other hand, doesn’t talk about the existence or presence of a cheetah or cheetahs in any place. It states that cheetahs, which are already known to exist on Earth, are the fastest within a particular scope. に doesn’t go well with such a statement.