I don't want to start getting daily plea deal offers from the prosecutor.

"First 50 defendants to sign up get 50% off their sentence!"

(1) Judging cases imposes huge administrative costs. Somebody has to pay those costs; if it isn't you, it's everybody else. Particularly with moving violations, where offenses are so widespread and the cost of admitting guilt is so low and where so many people have interface with the legal system, it makes sense to charge people who force costly adjudication that later proves pointless (which is what a discount for a guilty plea effectively does).

(2) The fines associated with most traffic violations are artificially low. Although any one person who speeds or runs a red light is unlikely in that one instance to grievously harm someone, people who drive carelessly as a cohort impose enormous costs on the rest of society, and those costs simply aren't priced into the $300 you pay for a 55-in-a-35 speeding ticket.