What is the point of Bo1? Why are so many people attracted to it?

I've been playing Magic for a long time and play semi competitive (competitive REL). So I've always viewed Magic as a game where sideboarding is a crucial part of the game. But as Arena has picked up I've noticed people really only talk about Bo1 (or that's what I notice). Why do people play this instead of full games to better prep for tournament play?

I'm not talking about casual or Freeplay or testing brews. Talking about Bo1 ladder and rank matches.