They did.
For Mongo going to postgres went great, but for complicated reasons we're stuck with Elasticache forever on our main product.
(This is a massive oversimplification, but still used rule of thumb.)
And, most importantly, lack of market availability. Nobody is going to sell you a relational database nowadays, and rolling your own is... daring. Postgres was the last serious kick at a relational database, but even it eventually gave into becoming tablational when it abandoned QUEL and adopted SQL.
But tablational databases are arguably better for most use-cases, not to mention easier to understand for those who don't come from a math background. There is good reason tablational databases are the default choice.
And most companies vastly overestimate their data, and believe it to be "big", when it could be trivially handled decades ago by server-grade hardware.