The defendant's choosing is so he doesn't get stuck with a [relatively] low paid and overworked public defender. It's so he has the same opportunity to hire the high powered winners on the state's dime and the state has to pay full price that attorney demands. If costs and taxes get too high, people should start voting for more selective prosecution candidates.
The state intends to take [part of] his life away, the state needs to pay for the process.