Some of the things we built in-house because it wasn't worth buying at our scale:
- data lake and supporting tools (we fired over 100 million analytics events in a single day).
- a/b testing software (assignment and analysis)
- support software
- email sending tech (we sent over a billion emails a month)
- hr tooling (the company hired in over 50 countries)
- payments integrations
- probably other stuff I don't remember
Basically, due to scale, these tools were interesting in their own right. Paying for them was either impossible (no off-the-shelf solution would be able to support it) or prohibitively expensive or both.