Diving 1v5 with no health?

I want to know why people feel the need to dive with no health vs a player or players with full health? ESPECIALLY when they have done it many times before and died every time! It's not that it's just one player doing it, it's when multiple team mates go off by themselves as far as they possibly can from everyone else to go against numerous opponents when it didn't work the first several times they did it! I main Soraka and I try to keep up with my team mates to heal them all but I can't be everywhere at once especially when I've used my ult on them already. People need to learn quick.