They do though, some just have a power level of 0. Do you really suspect that using something such as Ventus or Haos 2 (disregard the rulings/effects for a second) would be fair with a power level of 0? They even remade some cards giving them power levels. There was that one card that doubled a Pyrus Bakugan's G-Power for the next battle, essentially the Coredem card, but obviously it has no power level while the Coredem ability's power level is 4 which means it's more than likely to be the last card to be played. To further back up the point, the Coredem card is pretty limited in use whereas if they had re-invented the card to be used with Pyrus Bakugan, it'd need a power level of 5 to even be fair since its an automatic win unless on lowest G wins cards. I'm not sure that the card was removed from play either so if you could retrieve it using a Battle Gear effect it'd be even more broken. This is why they brought power levels into play in the first place, to help even out the game while being able to create any kind of abilities they wanted to.