I've been a sysadmin for decades, dealt with *nix based servers since the late 90s, yet for the most part I've used devices like Synology servers, simply because I don't want to have to manage technology to that degree at home.
I've built my own NAS when my last synology died, and I'm not sure I'll build one again. I've dealt with all sorts of issues that I just haven't had to deal with with a packaged solution, and I really just want to not think about that stuff when I'm not working.
Yes, I can absolutely do it for cheaper, better, and with more flexibility myself. Doesn't mean I actually want to.