60k for a single earner is a lot, way above average earnings. If you're going to compare like with like, then compare that earner to the guy next door earning 30k. Now, 30k still isn't bad, it's above average but it's only half of what the first guy earns. So Mr 30k may have a wife who also earns 30k, so yes the overall family income is the same, but there are two people working full time, not one, and there are likely to be extremely high additional costs- childcare, commuting, maybe 2 cars... (And yes, we've covered the fact that some people get free childcare from relatives but you can't hold that against them; that's the prerogative of the family, just as some people get gifted a house deposit, or given an interest free loan from family.
I can understand that on a surface level you might feel a bit resentful if you are a SAHP with a HR tax paying partner, but... I'm pretty certain it's been calculated that it would cost more administratively to put the cap on CB for combined earnings. So basically it comes down to do we want the govt to spend more money on administering a system just for the sake of a perceived unfairness? It's not going to put any more money back in the pockets of the SAHP family anyway!
When it comes to the examples people give on two earners on almost 100k between them getting CB, my own feeling is that there needs to be some consistency. We hear constantly about these high income husbands whose work life is so demanding that they need their wife at home, so we must therefore assume that two high earners probably means a hell of a lot of outsourcing, perhaps a nanny. All the families I know with two high earners have invested huge amounts in childcare over the years. And they are unlikely to have granny living around the corner... High earners have often had to move around and work their way up the career ladder.
Anyway. This is not a personal issue for me- no CB or freebie childcare here. And there is a lot to rightfully criticise in govt policy. But I think priorities need to be sorted. The UK is moving out of recession way quicker than other European counties and a big part of that is making work more accessible. Therefore childcare for SAHP is rightfully going to be low on most people's list of priorities. Those who work, or have children with complex medical needs have got to come first