It's not really because the developers are cheaper, it's because the vast reduction in complexity is cheaper. Let's say you've got a great general app idea and you're going to build a startup. Your app is going to have to be mobile and desktop. To do that well, natively, this means:

• you're going to need a backend dev who are probably going to be building APIs that are touching on web tech.
• You're going to need a developer team who can target Apple platforms, Android, and Windows. I lump Apple together here because although it's not entirely fair to say that it's as simple as they promise where you just click a box and your iOS app works on macOS, you're at least able to work in the same general toolset (Swift, SwiftUI, Xcode, etc.)
• You're going to need designers who can design to the specific needs of the platforms, which is also going to mean more domain expertise.
• testing for each of those platforms.
• This is true regardless, but you're going to have to deal with more platform-specific support. More platform specific documentation, etc. How do you do think x on platform y? Where is the button on this platform vs that one?
• maintaining feature parity as you continue to build is going to be much more difficult, and you're going to have to decide if you want to maintain feature parity and slow the whole process, or give up and launch on some platforms first (hopefully there is no one that uses a Mac and an Android phone or Windows and an iPhone or an iPhone and a Samsung Tablet or that gets annoying real fast.)

In short, moving from one platform to two natively doesn't double complexity and cost, it's far, far worse than that. It's not that a good web dev costs $70k vs an iOS dev that makes $90k, it's that a good iOS dev costs $90k, and a good Android dev costs $85k, and a good Windows dev costs $80k and one of those people hopefully is familiar enough with each platform to be the team lead so you can tack on another $20k for them...

And all the while you're building that team and building your 3 different platform native apps, a competitor or several will launch on Electron and web tech and take the market because no one except us nerds give a shit about whether something is using the right platform idiom or even knows what they are, and far fewer still have any idea how to check RAM usage and the like.