Since the grave is the end to them, I don't understand why they spend their relatively few moments of existence trying to convince others that they should join their group.
When you say "relatively few", what are you comparing that to?
I expect to live for about 120 years, barring any unfortunate circumstance. That doesn't seem like "few" to me, but rather, "much." Especially since it's all the life I probably get. In that case, it's not "few" but "all".
There's much living to do. If you can't shake the mindset that this is all a brief introduction to the infinite main attraction, you're going to miss a lot of life.
After all, what reason would there be to think and act otherwise if the grave is the end?
Why, pure, unadulterated altruism. Unlike you Christians, who have to have a reward in order to do the right thing, or a punishment to avoid wrongdoing:
Many hate any who would preach any other gospel than theirs, especially a gospel which preaches accountability for the way one lives to be reckoned with in existence beyond the grave.
See what I mean? I know it chafes that we're better people than you, Buz, but I wish you wouldn't take it out on us.