Because it almost certainly will cost MORE to means test it. The administrative systems required to run any means test scheme are much higher than for a universal payment. When it’s an ad hoc payment, rather than an ongoing benefit, it simply is not cost effective to means test it.
You are focusing on ‘fair’ as being about who needs it, but another view of fairness also considers what is best for the tax payer and public finances generally. Is it fair to run a system that costs all taxpayers more simply so people can feel, on a surface level, that it was aimed at people who need or deserve it?
The knee jerk, angry reactions we see here are actually the exact left-wing equivalent of the right-wing idea of the deserving and undeserving poor. The idea of people
in need and not in need is the same kind of logic and is based in similar moralised ideas (albeit politically inflected in different ways).
But, actually, the decision to do it this way was probably mostly about the most cost efficient delivery mechanism.
The moralised debate about who needs help/a hand out (depending on intention and perspective) has the added effect of diverting attention from questions about what the actual intentions of the policy are. Who or what is it actually aiming to help? Is that the right thing for the economy and public finances more generally.
Instead people are repeatedly invoking the figure of the £100k earner (who must be greedy, profligate and unaware of how luxurious their lifestyle is) and responding emotionally to the idea of them getting money they don’t need (or deserve).