How come the web’s compose text field doesn’t support native keyboard typing on iOS? Give it a try. You’ll notice that no autocorrect features work there. No capitalising the first word of a sentence. No spell check. No transformer model sentence completion.
It makes me feel like I’m typing with mittens on when I try to compose there on iOS.
The technical answer is because that functionality is explicitly disabled, as you can see from this HTML extract:
<div style="tab-size: 4" spellcheck="true" autocorrect="off" autocapitalize="off" contenteditable="plaintext-only" class="cm-content cm-lineWrapping" role="textbox" aria-multiline="true">
For the reasoning behind it, only @manton can tell us. Maybe he hates these features. 
Or, more likely, there’s a compatibility problem or something similar that’s the reason behind these choices. See, for example, this thread about an old issue between Markdown highlighting and the spellchecker.
I don’t hate those features. 
In the past there have been some problems getting these to play nicely with the syntax highlighting, but we should revisit it. We’re actually experimenting with some new web editor features right now so it would be a good time to try again.
Great! I hope you find a solution.