Composer is included in the "tools" category, of course. There are other tools used (ex: webpack, grunt, git and more), but I am looking at the stack (PHP + SQL) as the primary components on topic for this discussion. What I am not including are niche or fancy components that bring little or no value, add complexity or limits the availability of developers or long time support. We are also not writing code in Notepad (Windows shop by other people's choice, no comment), so it's not stone age.
We used in the past libraries that were abandoned, technologies that were big hypes for a year or two and disappeared, etc. In the corporate world you don't have one product with millions of users, you have tens of small apps that have thousands of users. Some are so simple I can write myself using existing (internal library of) components in a few days using basic stuff like Bootstrap (good enough), jQuery, Vue (nice one) and very carefully written SQL code (that is where volume and complexity exists).
Again, I am no developer myself, but I can write code when needed at least as good as the developers that I have from multiple suppliers.
In the end is cost versus benefits. We are very good at calculating both and in many cases the bean counting tells me to continue doing what we do.