FAANG companies have thousands of engineers working on their main products. At that scale, you cannot manage everything with that many people in one binary nor one database. Thus, business logic and data gets siloed into separate services which individual teams own hence the rise of microservices and different database systems for different use cases.
They also have billions to tens of billions of dollars worth of hardware costs per year so what can seem like a waste of time to optimize can end up saving millions of dollars worth of hardware a year.
They also have billions to tens of billions of dollars worth of hardware costs per year so what can seem like a waste of time to optimize can end up saving millions of dollars worth of hardware a year.