Single op-amps often come in 8-pin DIP packages, with 5 of those pins being standardized (or maybe it's just de-facto) like in the ones below.
(source: linear.com)
Older op-amps such as the LM741 (first image) have a very bad sense of what is 0 volts, so they require offset null pins to manually adjust them. Some newer op-amps such as the TL071 are much more accurate, but still make use of extra pins by having this feature, while their dual and quad siblings do not have this feature. Some modern single op-amps have three unused pins, and forego the offset null completely.
So why don't many modern single op-amps come in 6-pin DIP packages? They would still conform to the standardized layout, and fit in 8-pin DIP sockets. But they would take up less space in circuits designed for them.
edit: For examples such as TL071, I can that they would put the offset null on it despite it's absence in it's dual and quad siblings so that there's an option for those that do want to adjust it. But even then, many of these are available in different packages, so why not 6-DIP? That still doesn't explain it for the single op-amps that don't have offset null.
Best Answer
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.