From what you're asking, I believe you are making an assumption only based on your field of knowledge (guitars).
As a percussionist, I could say the same for guitars, as that's not my field, and to my eyes and ears almost all guitars are the same: sometimes they have slightly different shapes or colors of their bodies, that's it.
But I wouldn't, as I know that I know almost nothing about guitars, and that in reality almost any instrument has some amount of variety.
Another aspect that shows flaws about your assumption is that the guitar is technically a very simple instrument (which makes it easier to create variations), and is almost a "class" more than an instrument per se.
Also, consider that the above "simplicity" and the fact that guitars have been widely used in popular music both contributed to the development of variations.
If we consider instruments based on their "class" (not only their family), we can find similar variations, though:
- pipe organs: almost any traditional pipe organ is completely unique;
- drumsets: not only they are modular instruments, but each manufacturer has dozens of sets that are drastically different from each other, varying in size of shells, number, type and tone of elements;
- talking about percussions, there are dozens of types of cymbals of various size, sound and shape (and sometimes they are not even regular circles);
Going higher in the "class" concept, we can consider stringed keyboards: besides piano and fortepiano we have all the family of plucked keyboards (harpsichords like clavichord, spinet, virginal, etc). That grouping is not that different from your grouping of guitars, and even each one of those instrument has some amount of variety, including the piano: grand pianos have different sizes and ranges (the Bösendorfer Imperial extends to a full lower octave, counting 97 keys), and there are upright pianos that have smaller bodies and range, like those often seen in living rooms of american movies and tv shows, which are smaller and sometimes have a narrower range. And that's even ignoring some "custom" pianos that had limited production (such as the infamous left-handed piano, or both Klavin's creation, the huge Modell 370 and the Una Corda popularized by Nils Frahm).
Wind instruments, which are a bit more physics-specific (how the sound is generated) and somehow technically advanced, have their varieties also. While the variations are mostly based on their extension and reference key, there are actually different versions for most of them.
Take for instance the clarinet family, which lists all "standard" clarinets (the common Bb, Eb and bass clarinets), along with all their many tunings and other more rare cases (bassets).
The harp family contains the common concert harp (which has at least 47 strings) and the folk/celtic harp along many others, varying in size, number of strings, tuning systems.
Other cases: there are 9 types of saxophone; there are 8 basic types of trumpets (based on the tuning), but also variations on their construction; the oboe family actually includes 6 instruments; there are thousands of completely different synthesizers; etc.
Then there's the point of clarifying "how" an instrument is different from another. While some instruments are more generic (a "broader class") like guitar, others have a range which is narrower, and for various reasons:
- the complexity of its construction (which is also part of its sound);
- the uniqueness and popolarity of its sound (which means that there's more interest in having a specific sound, like the piano);
- the age in which the instrument has evolved and its repertoire (consider that classical music has also created a standardization of instruments, and since that repertoire is still played and popular, there's little interest in evolving instruments that are typical of that music);
- the price: guitars are generally way more cheaper and simpler to build than pianos, so experimenting with new ideas and actually being able to "release" a new "model" of guitar that has some market is much easier than trying to do the same with a piano;
But don't get confused by that "narrow" range: it's just a matter of perspective. In fact, each instrument, to those who know and play it, is sometimes drastically different from another one of the same type, even the simpler ones.
Even triangles, which are among the most simple instruments ever: a percussionist can spend hours deciding what triangle to buy. They are all metal, they are all triangular. But they are all different to us.
So, the variety is actually just perception, and mostly depends on how that variety is considered.