Because the Spectre is not all-powerful, all the time, no matter what he may say.
Despite his many appearances, terrifying speeches and particular periods where he seems almost all-powerful, the Spectre's actual powers fluctuate widely depending on the nature of his Human host and his relationship with his power's provider, the Supreme Being of the Abrahamic religions.
Jim Corrigan was a Gotham City Police Detective whose fiancee Gwen was kidnapped, He was guided by the Phantom Stranger on the instructions of The Voice. He leads Jim Corrigan to the abandoned warehouse where she's been kept, but this turns out to be a trap; Jim and his girlfriend are killed by the kidnappers and his soul is immediately imbued him with divine powers by The Voice and dubbed The Spectre...
While at his peak of power he has been known to challenge and even eclipse Superman, his powers are not always as effective, and are often downright unreliable depending on the psychological state of his Human host.
In some hosts, the Spectre has been shown to have incredible abilities, up to and including degrees of reality alteration, and in others, he has been shown to be little more than a ghost with a degree of mental or psychic ability.
The Spectre's actual title is the Wrath of God and was a weaponized aspect of God's rage, sent to punish those who transgressed against His laws in different periods. The Spectre's power is however eclipsed easily by beings such as the Archangels Michael, Gabriel or Lucifer who have been shown to hand the Spectre his walking papers with apparently NO effort on their part.
Different hosts have shown degrees of favor and as such display incredible abilities. Jim Corrigan, the most well-known host of the Spectre showed a wide range of power over his career before retiring and being allowed to enter Heaven.
Ultimately, different writers have taken the Spectre as a character and written to the power levels they were most interested in tackling. The Spectre has been involved in events of cataclysmic proportions in the DC Universe such as opposing the Anti-Monitor during Crisis on Infinite Earths.