looks like work on the android client started in 2011 (or at least, that's when it seemingly started using version control)

the app was released in 2014

so it has likely inherited decisions from ~14 years ago, I'd guess there is a several year gap where having a native desktop app was not even a concern

Also the smartphone landscape was totally different back then, QT's android support back then was in alpha (or totally nonexistent if the signal project is a bit older than the github repository makes it seem), and the average smartphone had extremely weak processing power and a tiny screen resolution by today's standards. Making the same gui function on both desktop and mobile was probably a pretty ridiculous proposition.