Because integration into the desktop is better as an Electron app. Eg sound and video calls, keyboard shortcuts, not having to worry about finding your Discord/Slack/whatever tab
Electron apps can use desktop capabilities. Web apps are at the mercy of the few desktop-bridging APIs that browsers inconsistently expose. They’re not talking about UI/UX “integration”.
Discord for instance has this “currently playing game X” feature. I have zero interest in broadcasting what I’m doing at the moment to the world, but many do and have this feature enabled. Good luck implementing that in a browser-confined web app.
I think it's because the apps have additional functionality and because the services push users to use the apps on their websites. Some of the additional functionality is artificially limited to apps as companies can put more tracking, advertising, and can ensure that people won't leave their service easily by just closing a tab.
Because browser's haven't built enough compatibility with the desktop to use it like a regular app, therefore severely limiting---sometimes intentionally---what you can and can't access on the file system. It is expected sometime in the near future that browsers will have enough sandbox protection that they will then enable app developers to do the same things that only Electron allows but without the excessive bloat you get from Electron versions.
* Steam (the root process, not the subsequent Chromium child processes) is 32-bit, as are a lot of games.
* Discord is 32-bit.