It's probably because they don't technically own it themselves. It's owned (and maintained) by the public... it just so happens that it's set aside for the exclusive enjoyment of them and their family in perpetuity.......
Having said that, I still don't know how they swing it with Sandringham, Balmoral and the very many other properties and assets that are decidedly their private possessions. Do they even have to pay tax on the £4,000 a week that holidaymakers pay to rent out the private properties on the Sandringham estate?
It does seem bizarre that they not only get far more public-owned opulent mansions than they could ever possibly need for their own exclusive use - without having to pay tax on the benefit like everybody else has to even just for a standard company car; but because they get these, that somehow means that everything they own is then considered in the same manner.
Almost as if they are automatically categorised as grand freeloaders par excellence, and thus not expected to ever know/do anything better outside of that remit.
I suppose people often complain about those who are 'in the system' - whereby getting one benefit or exemption to pay automatically qualifies them for many other benefits and arguably makes them significantly better off overall than ostensibly 'richer' people who have to pay for everything themselves. This is just the ultra posh entitled people's version of this.