It's not server-oriented. There are people who have used it as a desktop for 25 years.

Debian is the upstream for almost everything now. IMO, the more layers are added to Debian, the more delays or errors that can be introduced. For example, Mint was famously withholding security updates a few years ago. Ubuntu has snaps which have been consistently problematic ... enough that I switched to Debian on three systems because routine updates would routinely break Firefox.

Flatpaks have closed the gap. It is entirely possible to have up to date user applications on a stable, secure base system. Now that the Debian installer includes non-free firmware by default, the downstream distros are really just adding pretty wallpapers and color schemes. Not really worth it for the added overhead.

So I just cut out the middle man and go straight to the source.

Just my opinion, I've learned to appreciate Debian after 25 years of avoiding it and using every other distro you can think of.

Stable/unchanging (zero maintenance for several years, no new bugs appearing out of nowhere), as minimal or bloated as you want it to be (made my own custom live/installer ISO which means I can start fresh with my environment already set up in a matter of minutes), huge number of pre-packaged software, good documentation, identical base distribution across my servers and desktops, mostly sane defaults, community-backed (no or few corporate interests driving the project to shit... look what it did to Ubuntu/CentOS...).

Other Debian-based distributions don't bring anything of value to the table for me, I'd rather use OG Debian.

I use Debian as a desktop both for work and personal use because I am a boring person that doesn't need any surprises. I have been using it since woody after moving from Mandrake Linux

What makes Debian unfit for Desktop use? Nothing. Debian is as good a Desktop OS as Mint, Ubuntu, Zorin and any other distro out there. It isn't just for tinkerers. Perhaps it doesn't have the amount of new user friendly features but I would rather use an OS that pushes me to learn than pushes me to be complacent and also pushes me to seek the 1-click options rather than spending 5 minutes to set something properly. Debian might not be everyone's cup of tea... but for those who it is their cup of tea Debian will never disappoint.

I started out on Ubuntu back when it was basically just Debian with more stuff preconfigured and newer packages.
So I'm intimately familiar with the .deb ecosystem.
Moved away from Ubuntu when they started trying to push their own solutions over accepted standards (Unity, Upstart, Snap) and included ads in their OS.
Moving downstream to a Ubuntu-based distro felt wrong so I moved upstream to Debian instead.

I don't think it's the best distro out there in all respects, but it's rock solid, dependable, predictable and I know it inside out.
Plus, it lets you choose how up-to-date vs. stable you want it to be, with its testing and unstable branches, backports, apt-pinning and third-party support. And also, how bare-bones you want it to be, since it offers a minimal installation with less hassle than Arch as well as a full bells-and-whistles system with its default installation.

Because I'm a junior sysadmin. I need to learn all the stuff as fast as I can.

Well,

For me its simple: i use Debian for servers, so who not for Desktops either. And personally i dont consider Debian a "server oriented distro" perse. for me its basicly a kernel with userspace software. You can make it a server if you leave it after a minimal install, or you can make it a desktop if you install a desktop environment.