So, I'm someone who's worked in both custom application stacks and what functionally amounts to applications being built on top of CMSes. A lot of it comes down to the life cycle of the application and who'll be maintaining it.
For complex custom build projects, a lot of those have been in-house applications that had a full engineering team to maintain them. Meanwhile, working in consulting, where we're often transferring control of sites to a client team, we often build websites on top of a flexible CMS like Wordpress, or do a custom build with a CMS plugin (e.g. OctoberCMS for Laravel) such that it's easy for a client content team to maintain.
Some of it also depends on where you want to allot your time. For a while I had a custom database API setup for my personal portfolio website, but it was getting to the point where entering content was just a lot easier with an actual CMS (Craft, in that case), and the point was to have an up-to-date website of my work. CMSes make a lot of sense when a lot of the maintenance work of a website is adding and editing content (hence the name, I guess?), rather than changing functionality.
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