How widespread are custom/bespoke dtabase systems?

Are companies writing custom database systems from scratches (I mean "starting from zero" and designing+implementing systems that fit specific needs and employ data models that are more tightly tied to characteristics of data sets and interactions with these data) to tailor needs better suited to certain clients just a tiny niche tied to few companies or is it an actual booming "invisible market" lurking in shadows of obscurity that is not that much talked about "in the mainstream"? I can certainly see them being created for social media platforms or search engines. How about world outside of these two groups? Are existing popular "mainstream" database solutions enough in the "real work" or they are areas when need for more closely tailored solutions are blatantly obvious?