1. Find the site: not always easy. Still, same level of difficulty for webapps.
2. Find the HUGE download button: not easy, it's incredible how many people don't figure it out. Still, same difficulty as creating an account or logging in.
3. Download the file: might be blocked by security policies or corrupted by overzealous antiviruses.
4. Find the downloaded file: quite challenging for a good chunk of people, so challenging that mobile apps try to do everything possible to hide files to not scare users (ergo the app-centric OS models).
5.Launch the installer: see security policies, installer bugs, corrupted or out of date local OSes, plain old user error, memory or disk space issues.
6. Find the installed app afterwards: not an easy challenge for many people.
The desktop funnel was so long and fraught with peril that the vast majority of desktop users would get help from family or people they'd know. There were/are thriving PC help businesses around. Also, despite me being a PC fan, PCs could never reach ubiquity, unlike mobile devices, despite being the only option for 30 years and the web helping them with usability.
You're conflating "web apps" with "distributing through Internet" and forgetting that between floppies and web apps, for about 15 years the main way of getting applications was downloading regular pieces of software through FTP and HTTP ("download the installer" in Windows world, a package manager in Unix-like worlds, etc.).