He's "only" 60, which is kinda a young age to retire from acting. I think it's more just his career never took off (he never had more than bit parts) and he got a steady job as an academic.
IIRC Jeri Ryan had to keep secret before the big annoincement, to the point of having to wear a cloak whenever she left the trailer or the set during filming. Del Arco had similar restrictions. It would not surprise me at all if Brophy had the same constraints.
Lots of reasons. He might be busy with other job(s). Maybe it would be a SAG role but he's possibly no longer in the SAG (I'm not sure exactly how that all works). He might not be involved with the industry at all anymore. Or he might simply not be interested in reprising the role. Kor
Maddox should be in his late 70s at least. As a Starfleet cyberneticist, he was consulted when Data applied for the Academy in 2341 (58 years prior to this series). Brophy was only 29 years old when he (first?) played Maddox, and has been decried as miscast given his character's role in Data's already established long career (see Datalore from Season 1 that describes his career path). Brophy, who is now 60, would be a good age to play someone close to 80. Of course, I assume he pulled an Ira Graves at some point, and should only appear (as himself) in flashbacks.
And Del Arco is on or near the same level as Brophy, "importance"-wise, so that's a good comparison. If Del Arco has to go through hoops to keep his appearances secret, it makes sense for Brophy to do so as well.
Speaking as someone who has been trying to keep the curiosity over casting down to non-stalkerish levels, I find the absence of a public statement by Brophy or Star Trek about casting indicative. We have not gotten a flat denial, only a lack of statements or non-official statements. (I am certain that Brophy was aware the fan he talked to might share the news of casting.) This, of course, is not positive proof that Brophy will or will not appear. All I can say, especially keeping past precedent with Ryan and Del Arco, is that what we have seen is not inconsistent with him appearing. (I loved a tweet Ryan wrote after the news cake of Seven's return, about how it was so hard to keep the secret.)
Even in the days before I had the Internet, I thought "This guy looks way too young to have been in Starfleet when Data applied to the Academy." I rationalized it as maybe Maddox was in his 40s and he was in his 20s when Data applied to the Academy. Looking at the actor, his exact age was hard to make out, but I thought he looked on the young side. I'm glad to know Harry Mudd will make it to 2399.
What he said suggests he's not in it - but he is an actor, so he could simply have pretended. But he didn't clearly say yes or no, it only seemed like he didn't even know about it. So he didn't answer it and didn't lie either
Are people really clamoring to see Brian Brophy reprise his role? I didn’t even remember the name of the actor who played Maddox until just recently.
Kinda yes. To me Maddox is a really memorable character, it was such a pivotal episode. And I prefer to keep the same actors if at all possible. They did so with Hugh.