Same for most of us OP. I have two Specialized bikes for example I have built with Campy. In the town I ride, there are countless Specialized bikes but never have seen another with Campy.

So, if you like Campy, a good chance you will have to build it. Honestly for a guy who works on his own bikes, this is no big deal and with 11s speed, you can even leave many Shimano parts...like rear cassette, and their great cranks and brakesets that I also personally ride.

I prefer Campy shifters all said and really the only reason I ride Campy because I find the competition excepting shifter design to be as good...and in some areas like cranks and brakes...even better. A push on derailleurs...but hard to run Shimano derailleurs with Campy shifters without a Shiftmate.

As to why Shimano is ubiquitous and Campy isn't. This isn't Campy's business model. They aren't a high volume manufacturer and apparently they produce enough stuff to make a profit...they also make great production wheels I ride. They have been around for 80 years or so I believe and longer than the other guys.