Yes, a company of dropbox's size definitely has an iOS team (not 1 iOS developer). They absolutely have the resources to do this.
But I suppose you're right. Even true of Slack. It's totally a meme that Slack's electron client is horrendous and bloated, but it probably hasn't stopped their growth, so where's the incentive for them to improve it?
What sucks is that server-side performance is their problem, and client-side performance is your problem. It becomes their problem if growth slows or if users leave, but it's often too late.
I'm starting a company and writing native clients. There isn't any reason not to. There are lots of cross-platform GUI libraries that are fine for business applications, you get native speed, there are plenty of languages that are both safe and fast. You get full access to the full power of the OS if you need it. I think the current craze of non-native clients is because of web-developers who don't know how to do anything else, so they just mimic a website on desktop.
It depends what they're using. Often, the cross platform libraries are abstractions that just call the native functions for that platform. Even in that case it's difficult because differences to that platform can cause minor layout issues.
There are cases where a custom, bespoke application for each platform is warranted, but that is rare. It's a continuum between the custom, bespoke and a UI/color scheme completely custom to your app.
It seems no one bothers anymore, no matter the size of the company.
Any native clients we have now were built years ago and now they are only maintained.
I think nowadays even Audacity would be an Electron app.