Some tasks simply require more RAM. Compiling big software, for instance, wants as many CPU cores as it can get, and each compiler instance needs some amount of RAM to run efficiently. It's not unusual for a 32-core build to need 32-64GB of RAM to run at full speed. Work on a smaller program, though, and 16GB is absolutely fine.

Because they use one or two apps at at time, the ones they must spend all their time to perform their job. E.g. Excel and a web app to work on invoices and a stack of paper documents. I see 8 GB on Windows PCs too.

Containers is often the reason. You start a container and you are immediately pulling in a quarter to half a gig or more (often the latter).

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