Quick (TL:DR) answer: because both are great, both have their strengths and weaknesses, and both might be perfect for wholly different purposes.
So let me explain.
The upcoming Debian “Trixie” will have a kernel 6.12.something, while Arch has 6.15.7 while I’m writing this – might change real soon in a rolling release distribution. Likewise, Debian will have the ESR versions of Thunderbird and Firefox, while Arch will have the latest. Or let’s have a look at something for, say, composers – let’s choose musescore:
This was Arch, now let’s see Debian:
Ok, this is still “Bookworm”, so let’s see the packages site in a browser – same, musescore3. Same with other programs like Ardour, or for photographers, RawTherapee or Darktable.
So Debian packages are old, yes. That’s what makes Debian great for its stability, and this is something people like me often want. But on the other hand, creatives often want the latest and greatest, they might need the new features of the latest program versions…
… which is why I installed both. Simple. And free as in beer *and* as in free speech. 🙂
Oh, and in case you’re a creative: what’s the “pro-audio” group in Arch is the “multimedia-all” in Debian, called the Debian Multimedia Pure Blend. Easy, and without any 3rd party repositories (like the very good kx.studio).
And like always, thanks for reading.