So that "the same span of time being the beginning and end of the greatest good" would not necessarily mean that "life is the greatest good": but that - however we ourselves choose to feel our greatest pleasures possible to us -- that feeling will certainly occur between the points of birth and death.
And of course that hits on numbers of relevant issues such as not worrying about gods rewarding or punishing us, how important it is to seek pleasure while we are still alive, that there is no point of reference outside our lives by which to judge our goal of life, etc.
And there's still the issue that "good" can pretty easily be construed as "asset" rather than the next thing I am looking for that I already don't have."