Because the first effect of the card can be used as much as a player wants during their turn, the card provides nearly limitless material for summoning extra deck monsters, which makes Block Dragon one of the most powerful cards ever printed
The tactic is to try and reach this point as soon as possible, with Pokmon like Zeroara which start with an energy, or Magneton which can ramp
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cards that feature these iconic mythical creatures
One mana Unsummon or Twiddle with investigate would be great cards, both would still be decent at two and probably more playable than the one or three mana alternatives for the most part
Even with that, sometimes the only real explanation is that the anime has its own rules, and those rules are much more malleable than the real-life game