Because at some point after splitting your seed in to a number of secrets, those secrets either have to be displayed on screen for you to copy by hand or saved in a file(s) for you to distribute/print, both of which could be accessed by malware and transmitted to a third party if the computer has an internet connection. When generating a paper wallet, it should be done on an airgapped computer in case malware accesses your seed. When generating a secret, it should be done on an airgapped computer in case malware access your seed or secret.
I think the point that Abdussamad is making is that when initially creating your Electrum wallet... the Electrum seed is displayed on the screen... AND you have to then type it back into the computer to complete the process.
My issue isn't with Electrum, it's with what happens after Electrum has produced your secret. This should be done in an offline environment, and having a simple button to press that says "Generate a 3-of-5 secret for my seed" or similar will encourage people to do so on an internet enabled computer.
So, in effect... your issue IS with Electrum... as it is already doing what you're saying a plugin should not. And I would suspect that most Electrum users are NOT using it in an offline air-gapped way to generate their seeds.
Having a plugin to generate Shamir shares isn't likely to change that... and would not be doing anything or encouraging any behaviour that Electrum doesn't already.
NOTE: Don't get me wrong... I agree, people should be creating their Electrum seeds/wallets offline to mitigate risk... but I suspect the majority don't.
