Why can't "on Collision" have "Trigger Once While True" ?

I want to apply a "Trigger Once While True" to a "On Collision with Object" condition, why can't this be done ?

Because it already is

"On" conditions trigger once while true.

"Trigger once" is a short name for "run if the event is true this tick but was false last tick". That only applies to normal evaluated events which are tested every tick. Triggers and events like 'On collision' aren't evaluated every tick, they just fire whenever something happens, so "trigger once" does not apply to them.