Triac gate current understanding
When the triac fires into conduction, there will be only minimal voltage over it (these 120 V will be over the load (heating element)), so no significant current flowing through optocoupler. When power triac is off, there will be full line voltage over optotriac, of course, but no current as optotriac is off.

As soon as optotriac is switched on and the current through it reaches threshold, power triac starts conducting, voltage over it drops to one volt or so and there is no more significant current flowing through optotriac.