Different Operating Systems call it different things. Windows calls it Alternate. Even if it was only used when the primary was down, DNS doesn't provide any sort of guidance or standard on when to switch between primary and secondary. Is one query timeout enough to switch? How often do you reattempt to the first DNS server? When do you switch back? With individual queries, you can timeout and hit another NS server, but that's a lot easier at an individual level than to infer a global system state from one query timing out.

Why call it secondary then, that’s so counterintuitive lol

I don't think that's even the official naming. It probably comes from what Windows 95 called it back in the day:

On Linux, it's just an additional "nameserver x.x.x.x" line in /etc/resolv.conf, with no indication of which is the "primary" or "secondary".