Er, that's just objectively not true. Here's exactly one new FOSS desktop app: https://github.com/quotient-im/Quaternion - I don't know how old it is, but it's a Matrix client and Matrix is only 8 years old so less than a decade.
My idea when I installed Haiku was to make my own version of the "old computer challenge"[1], with an emphasis on using GUI apps.
Similarly to @probono (a FOSS dev), I also found Haiku "shockingly good"[2] at being a lightweight, responsive, easy-to-use desktop OS.
After some patching, I was even able to compile Tectonic[3], a modern LaTeX engine written in Rust, and Quaternion a Matrix client supporting E2EE[4]. All that running on a single core Athlon 64 and 1.5GB of RAM.
I posted some screenshots in a Mastodon threads if you are curious[5] (but my posts are in french sorry :/). And of course this comment is posted from Haiku!
[1]: https://dataswamp.org/~solene/2021-07-07-old-computer-challe...
[2]: https://medium.com/@probonopd/my-first-day-with-haiku-shocki...
[3]: https://tectonic-typesetting.github.io/en-US/
[4]: https://github.com/quotient-im/Quaternion
[5]: https://mastodon.tedomum.net/@tgoldoin/109554115997967651
Some examples:
Quaternion: https://github.com/quotient-im/Quaternion
https://github.com/Nheko-Reborn/nheko as a Qt telegram-like
https://github.com/quotient-im/Quaternion as a Qt xchat-like
https://github.com/manuroe/messagerie for instance is a SwiftUI client proof-of-concept that should work on macOS as well as iOS
None are as polished as Riot functionalitywise, but we will get there eventually - even more so if people contribute.