You can think of a turn-off as the point where something leaves the road it was on. A car is going straight down a highway; there is one gap in the barrier, and after it the car is somewhere else. Interest travels the same way. It runs along happily, then hits one small thing — a smell, a shout, a number on a contract — and goes off. In both pictures the word names the gap, not the journey and not the driver.
From the word’s history
Wiktionary's entry for the noun says it is 'Deverbal from turn off.' — that is, the noun was made directly from the verb phrase. 'Turn off' already does both jobs: leaving a road and switching something off. The noun names the point where either kind of turning off happens.
維基詞典在這個名詞底下寫著「Deverbal from turn off.」，意思是它直接從動詞片語 turn off 變成名詞。turn off 本身就同時做兩件事：從路上轉出去，以及把某個東西關掉。名詞 turn-off 指的就是那個「轉開」發生的地方或原因。
Wiktionary entry for turn-off (packet etymology_text: 'Deverbal from turn off.'), source_ref kaikki:3f6f69a190036ae4:1148470569
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