Reason? Well, the current progressive wealth tax code is a bit broken. It allows you to be taxed less simply by dividing your money up between accounts. I know heavy use of this would be prosecuted, but to be honest I'm a bit leery about enterprises like the Academy of Elwynn that have a separate account but that don't seem to be having a lot of economic activity. It also isn't very well defined for people with multiple citizenships and possibly bank accounts under multiple jurisdictions.
On the other hand, while merely being wealthy is hardly much of an imposition to the nation, holding a duchy is. It's traditional to try to mention every duchy somehow in pan-Shirithian culture; only having a duchy allows one to vote in the Landsraad; and duchies are also otherwise generally the most important kind of subdivision inside Shireroth. I think it would be well worth it if we returned to the system where each duchy gets taxed 2000 erb at tax day - not because we specifically need to be fair to the duchies (since we don't have voting by erb anymore), but because it's a fair trade between the duchy and the nation.
(there'd be more and this would be better written, but candlejack I have to le