Because any sharp cutoff where you stopped getting it would create poor incentives for people earning just below that cutoff.
What exactly are you objecting to, and what are you counterproposing? (If you're worried about having to mail two cheques, in practice it might well be applied as a reduction in tax once your taxes would be more than the credit. If you're worried about administration overheads, having every citizen able to claim the same amount exactly once is a lot simpler than means-tested benefits and means there's much less opportunity for fraud).
What exactly are you objecting to, and what are you counterproposing? (If you're worried about having to mail two cheques, in practice it might well be applied as a reduction in tax once your taxes would be more than the credit. If you're worried about administration overheads, having every citizen able to claim the same amount exactly once is a lot simpler than means-tested benefits and means there's much less opportunity for fraud).