I think whats actually happening is that the world have been overrun by an alien species called the Toad and they have taken over the human race! But they kept a few to do experiments and So Princess Peach was born! But Browser, seeing what the Toads have done couldn't stand for it so he "Kid Napped" Princess Peach to reason with her (as well as other things Fap). But the Toads could not let this happen and Browser and his army was too strong for them so they made Mario and Luigi to do their bidding for them!
And the only reason why we get to play as Toad in some of the Mario's is so that the Toads can make sure that the Gang isn't rebelling against them!
PC Specs: CPU: 7800X3D || GPU: Strix 4090 || RAM: 32GB DDR5 6000 || Main SSD: WD 2TB SN850
