Literally from the first line of the article you linked:

> Lyft has signed up to pay cloud market leader Amazon Web Services at least $80 million per year for the next three years, totaling at least $300 million.

You can launch a massive project, hire dozens of people, buy server and colo space across the globe, rebuild their infra from scratch, and get costs down to – what – $60 million a year? Lyft is closing in on $4 billion in annual revenue. A few million in savings is meaningless, especially when balanced with the massive risks of such a project.