The actual answer to the question is that it has to do with the output transformer. There is a certain amount of DC current you can put through the transformer before it begins to distort, so building amps with parallel tubes gets tricky as what you are doing is adding more current when you parallel tubes.
So most manufacturers in building a larger SETs go for more voltage instead. This is why you see the more exotic transmitting triodes used in the higher power SETs.
Regardless, the other issue that you run into with output transformers in general, but particularly SETs, is that the more power you go for, the less bandwidth you get. In most cases, the 7W of a 300b is about as big as you can get and expect bandwidth that might be considered 'hifi'. This is also why the smaller SETs, the 2A3s, 45s and the like have the reputation for sounding better- they have greater bandwidth. Now you can fudge things a little- optimize for bass or highs (and since most high efficiency speakers have troubles making deep bass, the tendancy is to go for the highs rather than the bass) since you aren't going to get both.
Push-pull overcame a lot of these issues to a certain degree, with its own set of trade-offs. For example, an SET will have nearly unmeasurable distortion as the signal level heads towards zero, whereas a push-pull amplifier will usually see increased distortion (this is commonly attributed to an additional circuit in P-P amps called a phase splitter; its worthy of note that not all P-P amps have to have this as a separate circuit). This is where SETs get the magical midrange that they are known for- when distortion is reduced, detail is revealed.
You still have the issue of bandwidth with P-P, but in general you can have about 10X more power available compared to an SET with equivalent bandwidth.
You also don't have the DC saturation issue, as the DC currents used by the power tubes cancel in the core of the transformer. On top of that, even-ordered harmonics are also canceled in the load, making for an amp that should be more transparent at greater power outputs.
So it may be that there are 20 or 50 watt SETs, which all battle trade-offs in their design, but a 200-watt SET is a rare thing indeed.
