A single ticket has to be printed and used in the space of a few hours. A monthly pass can be printed at any time and sold until the beginning of the month, for a much higher price. Much more risky for the transportation company.
Say you have a fancy printer and can print fake monthly passes. You can print a hundred fakes today "dated" November 23, valid for December 2026, and sell them until the end of November to people who can use them in December.
Ie. the end of the month November, not the end of December. I suppose you could also sell the remainder at a discount in December.
A single ticket has to be printed and used in the space of a few hours. A monthly pass can be printed at any time and sold until the beginning of the month, for a much higher price. Much more risky for the transportation company.