Taxation has overhead. If they were to actually track everyone's use and intention on a case-by-case basis, everything would get massively more expensive, just to offset the amount of extra bureaucracy needed to handle this.

It's the same idea as to why reducing the amount of means-testing and other hoops to jump to get social benefits would save taxpayers money - sure, more people who don't need benefits would get them, but that's more than offset by what would be saved by eliminating the workload of (and government jobs dedicated to) gate-keeping those benefits.