Ethereum's purpose is to decentralize components in the network. There is only one state trie in the network, Can't it be hacked and changed (because there is only one trie) ?
You have to distinguish between a centralised data model and a centralised infrastructure. State trie is a centralised model, but it is maintained on a decentralised infrastructure (Ethereum Network). Hence, you'll have to hack the network in order to hack the state trie.