Because it's public knowledge privy to anyone willing to do the research. As are the incomes of every other royalty or politician in European countries. Here in The Netherlands, the kings income is published on the official government website, for instance.
Also, they do not bring tourism. Unlike the pope in Rome, barely anyone gives a rat's ass about your queen. You don't see mass touristic influx when she's about to wave a hand from her balcony. Nobody cares.