They employ a bunch of coders who are really good at solving textbook CS problems and whiteboarding medium/hard leetcode style questions but lack the experience, interest, and in some cases ability to stretch beyond that into, say, proper systems or application development. There are far too many of these individuals who are able to "contribute" to the application code base more or less without the supervision of properly experienced and qualified engineers (there was some article, I think on Reddit, about how the app used to have thousands or tens of thousands of classes, re-wrote large bits of iOS core libraries, etc.).