There's totally a lot of crunchy CS in many things Google is into (and I do distributed systems stuff, so I can empathize heavily with your first paragraph). If you're doing it right, though, writing the crunchy CS bits of these should be exceedingly rare. Understanding is important, but I'm not sure you get a measure of working applied capability from writing this stuff in an interview. Discussing it, surely, but this is stuff that one--wait for it--Googles when dealing with implementations. And I strongly doubt that the overwhelming majority of Google developers are involved with the actual construction of this stuff.
The real reason, as near as I can tell, is because it allows management to not think as hard about allocating talent (the mythical "fungible engineer" nonsense), but whether it even works is another thing entirely.
Not everyone is constructing them, but everyone needs to be able to reason about they behave, determine what they need, and yes write it if a good option isn't available.
The real reason, as near as I can tell, is because it allows management to not think as hard about allocating talent (the mythical "fungible engineer" nonsense), but whether it even works is another thing entirely.