Because people don't value free things as much.

Why not make the fares free in Queensland? One reason can be found in the experience of the Miami Beach Transportation Association in the United States. The Association launched free shuttle buses along the coastline. However, the lack of fares led to a diminished sense of responsibility for the upkeep and care of the transit system, ultimately negatively affecting both driver satisfaction and passenger experience. Whilst passenger numbers initially surged, studies show problem riders resulted in raised personal security concerns as transit crime increased. Examples include increased assault, damage, and theft for users, becoming a deterrent for both new and existing riders. An attempt to resolve these issues was introducing a $0.25 flat fare, leading problem riders to avoid the service. Consequently, these negative factors began to rapidly decline, such as vandalism decreasing by 90% whilst passenger numbers remained steady.

https://ninesquared.com.au/insights/nearly-free-fares/