Because there's no mechanism for making it fixed value. It's worth whatever anyone wants to pay for it. And since it's decoupled completely from any sort of productivity (only existence), unlike a commodity-backed currency which is grounded in the value of the commodity, or fiat currency which is grounded in the economic activity of the country that issues the currency, and it depreciates at 1.25% per year, the only thing keeping it relevant is that banks are required to use it for international banking.

This might be enough, but I rather doubt it. Banks would, I imagine, rather quickly figure out how to use the currency only transiently to make transactions and then convert everything to local debt that was grounded in something that people cared about. The value of the currency could then be free to drift downwards to approach zero. And banks would probably lose market share to difficult-to-regulate alternatives like cryptocurrencies.

Again, I think the fundamental problem is that you do not create economic value by existing (or by laboring), but by creating goods and services that people actually want, which this doesn't do.

Also, on the off chance that the regulations can be made sufficiently onerous that the cost is worth it to be able to be able to easily do international banking, I also think it has a profoundly destructive impetus, which is to encourage countries to increase their populations as much as possible, all else being equal. So I think it's profoundly anti-environmental and, since overpopulation and resource scarcity are major causes of human suffering, anti-humanist.

But I very much doubt that it would get to that part, because the currency would devalue to the point where the tax was irrelevant and everything was pretty much business as usual except for a bunch of extra hoops you have to jump through to do international banking.