These big companies (including Google where I work) have so little idea of where they're going that they go way too general with interviews. If you want a good iOS engineer then ask them all questions about the platform and have platform experts interview them.
The concern is that priorities shift and this person might end up on a C++ backend in a year and you want to make sure they're good enough in general. The inability to forecast the existence of a role a few years down the road is a sign of incompetent planning. And having software generalists everywhere instead of domain experts is a great way to get good but not great software.
A lot of big companies (FAANG) do test platform domain knowledge, many also do platform-specific coding sessions. The reason why interviews aren't solely composed of platform questions, is that they want some measure of how good you'll be overall too.
Many engineers at big companies end up changing roles entirely (android -> iOS, iOS -> backend, engineer -> engineer manager, engineer -> PM, backend -> ML, etc).
Big companies don't just want someone who's knowledgeable about a single platform, they want someone who's also a skilled critical thinker and are able to break down hard/complex problems.
The concern is that priorities shift and this person might end up on a C++ backend in a year and you want to make sure they're good enough in general. The inability to forecast the existence of a role a few years down the road is a sign of incompetent planning. And having software generalists everywhere instead of domain experts is a great way to get good but not great software.