Because the UX of most email clients is extremely bad when compared with the webui of these email providers.
like what?
And probably a million other things that don’t hold up today.
Push notifications: https://github.com/freswa/dovecot-xaps-plugin
For server side filters I just set them up in Fastmail using the web UI. That's the type of action I do once or twice a year, so totally OK to hop on over to the web app for just that.
I have no idea what you mean by 'push notifications'. I have Thunderbird open on my desktop, and it shows me when there is email. I have K9 on my smartphone, and it shows me when there is email (I don't have it set up to display notifications, but that seems possible). That's basically all I need to do email.
(Also... Thunderbird is not that different from an Electron app itself. It uses Gecko instead of Blink, and has a few bits of XUL here and there, but the core premise is the same. Though it doesn’t use React at least!)
In mobile, webview-based apps exist mostly because they provide more ways to gather data on users.
If there was a way to run Mailspring on the web, I’d switch in a minute! I’ve tried to port it one time, but it got a bit tricky.
Mailspring uses a native module for syncing the mail, which would have to run either on a server somewhere or in a web worker. I think the web worker makes more sense nowadays, and would make it an offline-first app, but there’s a catch: how do you connect to IMAP/SMTP from a web browser?
Running the sync engine on a server is possible, but you have to have a server. It could make sense, though, if you’re running your own email server and want a killer webmail app to go with it.