Why do single op-amp DIP packages have 8 pins, rather than 6?
Historically, it was more efficient to put multiple instances of the same function into the same chip, so DIP packages usually had at least 14 pins.
The JEDEC MS-015 standard never had defined packages smaller than eight pins:



(Note that 10-, 12-, and 26-pin packages are also missing.)

With nobody actually manufacturing six-pin packages, the first one to do so would have had additional costs that would have been larger than just wasting two pins of an eight-pin package. In other words, creating six-pin DIP packages is just not worth it.

There actually are many (opto)isolators that come in six- or even four-pin DIP packages (e.g., 4N25, PC817), but the manufacturing of isolators is somewhat different (typically, LED and phototransistor overlap vertically) and therefore does not allow easy reuse for chips with 'normal' dies.

Packages other than DIP are commonly manufactured with fewer pins (e.g., SOT-23-5, SC70-5), so op-amps are also available in those.