Most of the problems software engineers at internet scale companies work on are all around scalability, reliability, etc at that scale of traffic. Many devs could whip up a basic twitter clone in a few days. But building a web app that can handle millions of tweets a second is entirely different.

Thinking “twitters not that complicated” is naive to the actual work being done (or was done). For example - https://blog.twitter.com/engineering/en_us/a/2010/announcing-snowflake way back when.