Bogen, R’ Shlomo Zalman Auerbach zt”l gave a psak regarding Shabbos clocks. As far as I know, he didn’t give a publicized psak regarding cholov stam (NOT CHALAV AKUM!)
R’ Moshe paskened regarding both.
As to why we follow R’ Shlomo Zalman regarding Shabbos clocks: I don’t know. I do know that every major posek now will tell you they’re ok based on the psak from R’ Shlomo Zalman.
Regarding cholov stam, the only major posek to pasken on it recently was R’ Moshe. Therefore, we follow his psak, as nobody argues on him. Yes, people are machmir, but nobody will tell you it’s treif.
It’s not people picking and choosing which Rav to follow, it’s pretty much every posek nowadays following the opinion of R’ Moshe with regard to milk, and R’ Shlomo Zalman with regard to the clocks.