Because SQLite is often the right tool for the job, more so than Postgres and MySQL. It scales insanely well and requires minimal configuration and near zero administration. I try to use SQLite for as long as is feasible in my projects purely because it "just works".

Don't let the "Lite" fool you. Depending on your needs, you can scale to 10s of thousand of users using just SQLite. Or not! It is all about knowing your system and properly evaluating your options. I choose to use SQLite because it often fits the use case of small to medium projects the best.