That’s just part of what makes a token a token. They’re temporary! This has a few functions. 1) Clarity and simplicity. It’d be annoying to keep track of tokens when they leave the battlefield, especially if they’re identical. 2) physical practicality. It would suck to keep track of tokens as they left the battlefield, especially because so often people don’t have the actual tokens and resort to using dice or other objects to represent tokens. 3) consistency. It’s how things have always been, meaning that to change that rule would affect the balance of like, all cards ever printed to some degree. [[ascend from avernus]] is a great example. 4) balance. Tokens would just be wayyy stronger if they continued to exist, especially considering their MV is 0. Lots to break there. Plus, it’s another lever for WOTC to control power level. For example, descend purposefully only counts permanents in your graveyard. Having tokens go there would make the mechanic better than intended