In a stark contrast to the eagerness of fiends to make deals, the celestial servants of the Seven react to pleas for power with perplexion. Is it not the sacred duty of those chosen by the divine to carry out Their will? One who actively seeks the power of the divine is likely a pretender to divinity itself.
As a result, Celestial warlocks are few and far between. Most often, they are the travelling companion of a cleric or paladin, inspired to follow the same cause.