So are there a lot of other formats online, or are there only a few worldwide sellers that produce other sizes?
If I view your question literally, I think it is more complicated than being either one or the other. I think, however, that I understand what you’re getting at – why do so many of the solderless breadboards we see have five-hole electrical contact strips as the rows? [I guess you found a few with six-hole rows.] Example here from Guide to Solderless Breadboards
The answer to the first part of the question, “are there a lot of other formats online” is; with respect to the characteristic in question, no, there are not. If there were, you would likely have already found them or others would have quickly pointed them out.
That begs the question, why not?
While I am certain that I am not a solderless "breadboard historian", I am, apparently, willing to try to address the question.
I believe that the integral issue to understand is the evolution of the concept of a breadboard. The name comes from the similarity to wooden breadboards used at the dinner table to hold the loaf of bread for cutting and serving. So, why not also use something like those, which were cheap and available, to assemble electronic components – and folks did, like this one, from The Evolution of Breadboards
From there, it seemed reasonable to start producing prototype boards that had patterns to accommodate soldering the components on the boards and especially DIP ICs, as they became widely available and popular.
Then, we get to the “solderless” breadboard, which seems a logical extension of the previous prototype solder boards. In a sense, I think that the onset of the popularity of the solderless breadboard also correlated with the continued increase in availability of the DIP IC. The advance was that they were not intended to be permanent - you could make multiple attempts at constructing the circuit without going through the soldering part - an extension of the plug board.
Having four connection points for each pin was enough for most, but not all, situations the customer might encounter. Similarly, the width of the separation between adjacent rows was intended to accommodate the width of many of the available DIP ICs, with even the larger pin ICs leaving a few connection points.
Take a look at the Wikipedia entry for Breadboards and the list of patents – especially the the last one from the early 1970s. Maybe that one gave rise to the beginning of the pseudo-standard that we see so frequently.
So why are you feeling left with the alternatives of having to “3D print or cut breadboards”?
Perhaps it is a simple matter of being caught between principles of "Necessity is the Mother of Invention", giving rise to the solderless breadboard initially, and subsequently leading to the current pseudo-standard ; and "Customer Need is the Impetus for Design Evolution" not [yet?] resulting in low cost alternatives of infinite variety.
In other words, given the various ways folks come up with to address the need when it does occur, e.g. cutting solderless breadboards to accommodate the width of some embedded controllers, or just using the availability and variety of prototype solder boards, and female header sockets, it may be that the need is simply not great enough for a manufacturer to make a profit.
That’s what I think anyways.