Probably because the actions required to get the pet back don't fall under the definition of restful...It takes 8 hours to get the pet back. That’s also the same length as a long rest, right? Why are you saying it takes 16 hours?
...therefore, like it or not, the math as presented is right: 8 plus 8 does come to 16.Man, your math skills are getting worse.
But @SkidAce and others do have a good point: why not just revive the flippin' thing? It's automatic in 5e, and nowhere near as costly as earlier editions.
That said, my thoughts keep wandering back to this:
Pirates get to have parrots on their shoulders. Rangers get stirges on theirs; and I don't know 'bout you but I sure don't want a stirge that close to my neck all day no matter how well traied it is...