The simple answer is that if you're working on a growing product, your engineering team is probably stretched thin. The why behind all these questions is most likely a short term optimization of time.

A $4B "valuation" isn't particularly relevant. It's just a matter of the actual (not potential) resources they had at the time, and how they were allocated. And I'd bet that was mostly allocated toward product growth and development, not stability. Because, well, of course it was.