Mainly due to them being both more tightly integrated to C++/Python than JS/TS for building desktop apps.
Yeah, but, honest question, why would you want to use JS if you didn't have to? It's like, the worst language possible.
If you use typescript, the tooling is great and the parent also mention it so I assume they are using it.
Code reusability between browser/desktop/mobile is one. Easier to find developers, faster development speed due to ecosystem, previous experience/familiarity etc. I guess.
Maybe it is just your subjective opinion? I am pretty sure that there are people enjoy writing JS/TS over C++/Python.
I would pick writing JS over C++ any day. Of course, that's just my opinion. I am sure yours differs.
JavaScript is actually not too bad. It lacks static types, but Typescript gives you one of the better type systems. But I'd take JavaScript over PHP or Bash any day of the week and twice on Fridays.
If I were no experience for GUI development, I'd prefer to learn Web tech than Desktop tech like Qt because it look like more popular and versatile skill.