Because those tokens do not attack, they ENTER ATTACKING. To attack with a creature means to DECLARE it as attacker in the declare attackers turn based action. Same goes for any other creatures entering attacking, like via Kaalia of the Vast. The CR even spells that out explicitly:

508.4. If a creature is put onto the battlefield attacking, its controller chooses which defending player,
planeswalker a defending player controls, or battle a defending player protects it’s attacking as it
enters the battlefield (unless the effect that put it onto the battlefield specifies what it’s attacking).
Similarly, if an effect states that a creature is attacking, its controller chooses which defending
player, planeswalker a defending player controls, or battle a defending player protects it’s attacking
(unless the effect has already specified). Such creatures are “attacking” but, for the purposes of
trigger events and effects, they never “attacked.”