Earlier quoted context omitted.
If you only tax money people don't spend, i.e. only tax savings, then you discourage saving. Taxes aren't just about fairness, they're about setting the right incentives. Do you think discouraging saving is a good incentive?
I agree it would be problematic to tax savings. However, if you have a two earner household a nanny is arguably a cost of revenue. I could imagine a system that allowed cost of revenue expenses to be deducted but not, say, a trip to bora bora.
You actually can deduct some childcare costs with a Dependent Care Flexible Savings Account. It's only up to 5k a year though which is lower than the cost of even the cheapest daycares in many areas.