Because it's an effect from another model that happens "in-game."
It would be like saying because Hoffman could assimilate Borrowed Technology, he should always be hire-able by Ramos. (Little more extreme example, for hyperbole sake)
Hopefully to clear some things up, different effects take place at different times:
Pariah is an ability that has an effect during crew selection. Why doesn't it interact with Embrace Death? Pariah does become active during crew hiring. Embrace Death however becomes active later (when being affected by talents or spells that affect the undead.) Hiring, as far as I know, isn't a talent.
Guild hounds have an ability that takes place in crew selection, I believe its "Trained in Pairs." Guild Kennels takes effect at the "start of the encounter." So, after hiring as you are starting to shuffle your cards is when you magically receive your soulstone for hiring two of them. (This is how it was explained to me, I could be wrong. It works kind of like a tax refund.)
Other masters have broadened hiring abilities that influence hiring crews. How is this limitation any different by timing? To use the Hoffman example at the top, it's something that can only come up during the encounter, so it can't affect hiring choices pre-encounter.
Coryphee have a limitation on what faction can specifically hire it, as an ability. Most models have a limitation, not written on their card, as to which factions can hire them... faction restrictions. The Coryphee has the reverse of Levi, it is a true hiring restriction as I think of it. They can only be hired by crews containing an Arcanist master, so Hoffman, despite his Arcanist ties hiring ability, can not hire them.
To the same extent, going the opposite direction, the Hunter and Watcher have Borrowed Technology allowing them to be hired by crews containing Ramos, who has no Hiring r
There's even Lucius, who has an ability upon hiring, that backtracks to determining strategy, checks to see if you used a soul stone to reflip, then adds one SS to your cache when you determine the size of it. This one has also been a doozie to get my head around. Add Lucius to your crew, after re-flipping, gain a soulstone. This one, like the Guild Hounds ability, takes place as you are shuffling your cards. I read this as "Lucius already knew you burned the stone and like the great guy he is, showed up with an extra one," it is called Advance Planning after all.
So, the moral of the story is that everything takes place at different times and is just always going to be really confusing *grins* Basic guideline is "Cannot always overrides Can, and theory never overrides anything."
The way I view masters is that the first one is chosen, then the second one is hired. This just keeps the confusion down of wondering if you hire the first one, how did you hire it. Is the Player in effect a deity? Who hired the player? (Is the player omnipotent? Does he know what I did last summer?)
Hopefully this clears anything up. I could also be wrong on some things, so feel free to correct me.