Bingo. I recently left my job at a FAANG, and there are the top three reasons I saw for code to be written locally.
(1) Need to interact with other internal systems.
(2) Pure NIH.
(3) Need to scale further than outside solutions.
1 and 3 are closely related. There are quite a few legitimate category-3 internal services for provisioning, configuration, service discovery, monitoring, upgrades at various levels, fault remediation, etc. Any other production service would have to interact with most or all of them. It's often easier to build something local than to add all of those "touch points" to an open-source project. I know because I did both while I was there.
But pure NIH is very close behind as a reason. Despite all protestations to the contrary, engineers get far more "impact" for creating new things than for fixing old ones. It's hard to get somebody to do X when their bonuses and raises are better served by doing !X. This ends up amplifying, instead of attenuating, the natural impulse of all engineers everywhere to build new things because it's more fun. People always make up other reasons, and perhaps even believe those reasons themselves, but nine times out of ten those reasons are pure delusion.
(1) Need to interact with other internal systems.
(2) Pure NIH.
(3) Need to scale further than outside solutions.
1 and 3 are closely related. There are quite a few legitimate category-3 internal services for provisioning, configuration, service discovery, monitoring, upgrades at various levels, fault remediation, etc. Any other production service would have to interact with most or all of them. It's often easier to build something local than to add all of those "touch points" to an open-source project. I know because I did both while I was there.
But pure NIH is very close behind as a reason. Despite all protestations to the contrary, engineers get far more "impact" for creating new things than for fixing old ones. It's hard to get somebody to do X when their bonuses and raises are better served by doing !X. This ends up amplifying, instead of attenuating, the natural impulse of all engineers everywhere to build new things because it's more fun. People always make up other reasons, and perhaps even believe those reasons themselves, but nine times out of ten those reasons are pure delusion.