The issue isn't that Linux lacks proprietary applications. Its that Linux development focuses on apps that appeal to people who write code and this means Linux has a highly technical userbase that prefers open source software.

If Linux desktops had a critical mass of users that were willing to buy proprietary software, the proprietary software companies would make Linux software for the same reason that game development companies make games for Nintendo's underpowered gaming hardware if it sells well (but don't if it ends up flopping like the Wii U).

To get to that critical mass, Linux desktop needs something that Mac and Windows don't offer that appeals to the average non-technical computer user. The single best opportunity for Linux was when Microsoft came out with Windows 8 with its awful new "Metro" interface. If a Linux distro had a GUI that was at least as good as Windows 7, out of the box support for Windows applications, drivers that just work and major OEMs willing to sell it preinstalled at the time, it would have drawn Windows users. Right now, thanks to Valve's efforts at getting Windows games to run on Linux, the Steam Deck is likely to be the most commercially successful GNU+Linux computer ever made because it can run most of your Windows Steam library and gaming is the main reason consumers buy Windows these days (the needs of low end consumers who just want to read email and be spied on by Facebook are best served with an iPad or Chromebook).

Linux is, on the surface, a much more appealing platform than iOS or Android for developers because there's no monopoly app store taking a 30% cut and demanding you take features out of your software. Its also more appealing than Mac because there's no Gatekeeper program that you have to take extra steps to get around. Developers tolerate these practices because the users are on those platforms and especially because Apple products tend to sell to wealthier people who are more willing to spend money on software than Android or Windows users (let alone Linux users who are generally believers in open source and some of whom are "free software" advocates who agree with RMS's views on the immorality of proprietary software). Ultimately, software companies will go where the users are. Platforms don't have to cater to them. They have to attract the users which forces the software companies to release their software for the platform. When you focus on making a platform that appeals to software companies instead of users, you end up with the Epic Games Store which has lots of software but few users.