Because there has to be a legitimate reason to report someone.
The system doesn't go, "this guy was reported, he must be doing something wrong"
It goes, "This guy was reported for X reason, let's check the legitimacy of this report before adding a mark to this player's record"
After a certain amount of marks, the player is banned for a set amount of time depending on how many marks of X reason and the degree they violated that rule.
False reports only serve to reduce the legitimacy of your future reports, and considering how premades are allowed you only serve to hurt youself by false reporting.