This was the way all synthesizers were designed in the beginning. Soon some manufacturers discovered that it was more commercially successful to pre-package the synthesizer in a certain way, locking it into a single patch with just a few choices available to the musician and attaching a piano keyboard to the circuit. The word "synthesizer" quickly became synonymous with this design, often even called "keyboard" and used almost exclusively by pianists. Most people out there are not even aware of the modular synthesizer design. In the process of commercializing the synthesizer its abstract sound world had to be mainstreamed by letting it imitate acoustic instruments - strings, brass etc. Synthesizer quality was measured in terms of how realistic its sounds were.

The synthesizer is so much more. Why not let the synthesizer have a voice of its own? Liberated from the "basic patch" the synthesist can explore new worlds of sound the way this instrument was intended. The synthesizer is truly a new instrument and it is in several ways unique compared to other musical instruments. Besides being an electro-acoustic instrument the most striking difference is that the synthesizer doesn't impose the method of instrument control on the musician. Naturally some use piano style keyboards, there is nothing wrong with that, but it is just one of many ways of controlling the synthesizer. So while traditionally instrumental skill and virtousity is largely based on muscular technique on a common "controller" (strings, keys, sticks etc) synthesizer skill is based on innovation. This makes it harder to define synthesizer virtuosity, as it is not relational. It has no defined limits.

Some instrument builders such as the innovator and pioneer Don Buchla has successfully avoided the commercial side of the synthesizer and continues to this day to break new ground for the true modular synthesizer. His 200e Electric Music Box combines the best of two worlds.
The modular synthesizer has gained some interest these last few years and a number of new interesting designers are emerging. But the keyboard still seems to be the controller of choice for some reason...
The world of computer software has opened a lot of new possibilities as well similar to the idea of the true modular synthesizer. Unfortunately not many developers seems to take advantage of this fact. Most software synthesizers are very limited. But there are exceptions, for example Supercollider and MAX/MSP.