It's because Google's culture is engineering focussed while pretending to be product focussed. As a result, Google breeds great engineers but not great product people. Also, their promotion process shamelessly promotes peer cannibalization. So, its easy for naive people to stagnate.

Source: Ex-googler.

I think Google just can't find people internally who want to do it.

I know of two groups (one of maybe 10, and another maybe 30 engineers) that sat essentially leaderless for several months each before Google hired a manager from outside the company. Neither one of these, in my opinion, have a good handle on Google culture.

There is no real reward for becoming a manager (no more pay, no more level, no more nothing), but a lot of hassle, so very few engineers want to do it.

At least that is what I see from my limited vantage point.

Companies typically hire outside people for leadership roles when they want to change their culture or gain some new industry connections. Promoting someone from within will reinforce the existing culture, for better or worse.