It is because not all enumerable objects have the last element.
The simplest example would be:
[1, 2, 3].cycle
# (an example of what cycle does)
[1,2,3].cycle.first(9) #=> [1, 2, 3, 1, 2, 3, 1, 2, 3]
Even if the enumerator elements are finite, there is no easy way to get the last element other than iterating through it to the end, which would be extremely inefficient.