"Why wouldn't you travel by main roads?"

Episode 3, Betrayer Moon - Triss

So, let's talk about Triss!

Over in Yennefer's side, Istredd tells us, "Stregobor says Temeria leans heavily on its mages." Triss says, "After Nilfgaard overthrew their king, the Brotherhood couldn't risk it happening again, so they sent me here three months ago to cure the creature."

The striga has been active for six years, and the curse killing her mother happened fourteen years ago. Temeria really likes mages. Triss has only been there three months.

So. What's up with that?

Istredd was supposed to go to Temeria but doesn't back when Adda is a kid. Either someone was shifted from a different assignment to take his instead or they didn't assign a mage to them until later on. And in the other direction, we'll later find out Triss knows Yennefer, apparently from school days together, which means she hasn't just graduated herself and must have had other postings before this.

We don't know how long mages stay at a court. Yennefer fucks off from her job a couple years before Geralt gets entangled with the striga (again: Yennefer is not behind the striga) so it's possible for court mages to stay at a single court for some time, but we don't know how representative that is. The effort Yennefer went through to get that exact posting means she wouldn't have been looking for a chance to transfer somewhere else, and it seems by the time she realizes she's unhappy she's realized she's unhappy with the whole system and that switching positions wouldn't matter.

So possibly it's normal for court mages to transfer around a lot, or possibly Triss is unusual. It's a shame Geralt apparently doesn't pay much attention to this, because this seems like something he'd have commented on if only he had an opinion and then we'd have some idea.

I think there was probably another mage who missed the initial curse (or let it happen), then found themselves unable to sway Foltest once the killings started. Triss' phrasing sounds like she was sent to a mageless king rather than that she's replacing someone else, so I think the most likely circumstance is that Foltest did have a mage but kicked them out for insisting something be done about the monster and only accepts another one only because he's promised they won't harm it. Back in Renfri's episode, it certainly seems likely Stregobor got kicked out by the king for wanting to kill infant Renfri and this episode echoes that one. And Triss at least claims she was "sent" to cure the creature, instead of deciding this herself when she got there. If Foltest rejected the Brotherhood for wanting the monster dead then he'd need to be told they were sending one to try to cure it to agree.

(Worth pointing out is that the king is a terrible, terrible king. This is not a situation where there's unrest due to outside factors. Just like with the Nilfgaard king the Brotherhood also wanted propped up, everyone would be better off with the guy gone from the throne. In the books, the king at least tries to manage the death toll by putting criminals in her path and is actively seeking a solution the whole time, and although he does value her more than the people she's killing, the other issue is when various people have tried to kill her she ate them. Here, he's been completely ignoring the problem (including even that the striga is herself suffering) and the Brotherhood is propping him up anyway because either they've drunk the monarchy kool-aid and somehow delude themselves into believing divine right is a thing or they prefer stasis no matter how much suffering there is. A mix, maybe.)

Now, Triss does seem to be invested in curing the kid, but the emphasis she puts when explaining is that it's Foltest is going to boot her right out of the kingdom and never let mages back in if the kid dies, ruining the Brotherhood's plans.

And it also seems Triss does not even know if it can be done.

She first says, "Two thousand orens if you can tell me what exactly killed these people." That might just be that she wants to test Geralt, but the conversation that follows establishes that unless she is a superb actor committing to this for no discernible reason, no, she has no clue.