But I'm a privacy and self-hosting nut, which is probably just another way of saying the same thing.
(I will probably never actually do it, but that doesn't stop it being on the list).
Sure, we can shell out to libraries and to other people's work, but at some point, you will have to understand the thing that you've abstracted away if you want to evolve it.
Or as the kids say, let him cook: https://knowyourmeme.com/memes/let-him-cook-let-that-boy-coo...
It can be more or less translated to "I want to see the outcome of what they're doing" usually used after the person who is "cooking" is criticized for their methods.
plenty of folks dabble in art, many of which quite poorly, when good and even great artwork can be purchased for not all that much.
some paint the walls of their house rather than call a professional painter.
there are countless reasons, including but not limited to the easiest, most flippant, and possibly the most human response to your question.
why not?
Because you're able to offer a product with differences in the market.
Because you can.
It's not like they're implementing their own DB to get to a MVP of their product "Tinder, but like for dogs".
Well as it happens, https://explore.marginalia.nu/init
anybody or any team that sets out to build something will prioritize different things and end up with radically divergent implementations.
Because by not being a huge team you can do it better.
Read any articles about how amazing Google's search is lately? Me neither.
I'm building stuff myself because it's the only way I'm aware of to run a search engine capable of indexing quarter of a billion documents on a PC.