Because it's not an unlimited, infinite resource. They are physical systems with unsolvable bottlenecks, and as soon as you release pressure in one place, another crops up. By making it absolutely free, the cost to a rider for an inconsequentially short trip is zero, but the rider using it for that short of a trip makes the system as a whole less usable for other riders because the system is full at a chokepoint. This happened in Boston on the Green line. Riding the T was free for students, but the number of young healthy students using that perk to ride 1 station overwhelmed the system, causing it to be harder for everyone else that had to use the system for commuting. I support better access to public transportation, but free has issues is its own.