Exactly to the point: They can implement E2E encryption in-browser, on dedicated desktop apps, with supported routers, and basically anywhere. Even $5 microcontrollers can generate and use the same encryption protocol.
But they don't because modern phones have (semi-)secure enclaves that can hold encryption keys and protect them from most hacking attempts. Desktops and browsers lack this, so any conversation you have via these other platforms in the computing environments of 99.999% of the population (please spare me the 'I use qubes, so ha!' speech) has a much lower level of security/privacy. Since most people want conversation sync among the desktop and mobile versions this means your security drops to become the lowest common denominator among all platforms.
It can be done, but it shouldn't be done if you actually care about security or privacy.
- That mobile can also be false security as "0 days" are currently in the wild and mobile phones are typically always online
- Etc.
If you truly want security, there's a really compelling argument for live-booting a distro like https://tails.boum.org/ and then rebooting when you're done. On the other side there are compelling phones such as the Libre 5 (assuming there are no current 0 days).
Tails and purism phones are the same 'I run qubes' fantasy that I expressly ignored. No one uses these, and they are not going to ever use those systems. There are fewer 0-days and CVEs in the mobile environment and for at least the next five years or so the mobile environment will always be more secure than desktops. Right now the single biggest step any 'normal' person can take to secure their digital life is to throw out their desktop and live completely on mobile devices and consoles for gaming.
Unless you're buying a new flagship every 2 years you're probably not meaningfully more secure than you would be on a desktop. And buying a new flagship every two years is probably less realistic for most people than running purism or Tails. Those at least only will cost you time.
