Because Lego has a monopoly, built on people not knowing the alternatives - and, in fact, the original inventor, Kiddie Craft, too - and using legal loopholes to sue anyone however possible.
Other manufacturers have long since overtook Lego in terms of quality (especially color consistency), quantity, love poured into design, licenses - while sometimes being a third of the price, but always cheaper. Including Cobi, who exclusively produce in Poland, while Lego also produces in China. One part of Burg Blaustein/Blaustein Castle (made by BlueBrixx) would probably cost as much as the whole thing if it was made by Lego!