That's an excellent question.
In fact in 80% of the cases where employees receive free parking from their employer, this is a non-taxable benefit. That is because, by law, there is a clause that states that if the parking spot is not reserved or guaranteed, it is not taxable.
So, an inequity currently exists between the tax treatment of the parking benefit and that of public transit. There is nothing to encourage employers to provide a supplement for public transit.