Triskaideka vs. Dekatreís

Triskaidekaphobia is the fear of the number 13.

Paraskevidekatriaphobia is the fear of Friday the 13th.

Why isn’t the fear of Friday the 13th “Paraskevitriskaidekaphobia”?

What determines the appropriateness of “Triskaideka” vs. “Dekatreís”?

Is the latter modern Greek? Vs Classical Greek for the former.

Endeka (11) stayed endeka throughout history.
Dodeka (12) stayed dodeka.
Everything after 13 switched around when classical Greek evolved into modern Greek: triskaideka (13) became dekatreis; tetrakaideka (14) became dekatessera, etc.

For those so afflicted, would a 12-step program be advised?