Two comments under this question include:
This might be a rhetorical question and unanswerable in its current form. It might be better to ask "why is this understood to mean 'seven lifetimes'?" or to ask "Since when has it been understood this way?" or "Are there other ways to interpret this phrase?" or something like that.
Why might it be unanswerable? Peoples comfort level and actual Dhamma truth are two different things.
Why might it be unanswerable??