Does anyone but Dr. Soong make androids in the whole of known existance? How the hell was this absurd happenstance even arrived at? A living Hologram, I don't even want to get started on, it's just too magical for me to wrap my mind around: Only one part of the ship's computer would be sentient - capsulized in a single hologram character? I just don't get it, but I do like The Doctor on Voyager, very much. What I don't like is Dr. Soong's corner on the android market. It's annoying ...
No, Soong was not the only one who could build androids. Dr. Korby in the TOS episode "What Are Little Girls Made Of?" built 1. Ruk, a more primitive android that looked a lot like Lurch from the Addams Family

2. Andrea, an attractive human female android almost indistinguishable from the real thing. 3. Dr. Brown, an android assistant to himself who may or may not have been built to replace a real person. 4. a fairly convincing replacement android for Kirk that he sends to try to take over the Enterprise. 5. And it is ultimately revealed, an android body that he copied himself into before his organic body died.
The fact that they NEVER followed up on any sort of connection between Data and Korby's androids, even if just a mention in passing from Data one of those times he was researching himself and what it means to be human, has always seemed like one of the biggest continuity HOLES in Star Trek to me.