Urza saw the wall and realized that even if he tore every Phyrexian to pieces, they would still resist him.

Dominaria Remastered

Wall of Junk

Defender
When Wall of Junk blocks, return it to its owner's hand at end of combat. (Return it only if it's on the battlefield.)

Defender (This creature can't attack.)
When Wall of Junk blocks, return it to its owner's hand at end of combat. (Return it only if it's on the battlefield.)

Urza's Saga

Whenever Wall of Junk blocks, return it to owner's hand at end of combat.