It doesn’t detect the passive voice, punctuation errors, or tense mismatch. While spelling catches many, it doesn’t catch them all.

That sounds more like you need to work on your grammar. One should be able to do those things on their own

I, for one, don’t really care if someone has written to me in the passive voice. Or is changing tense. The message is still received. If it will mean they are more secure that way, then I will allow it.

Ok, but others want to correct those issues,and are specifically asking for ways to do so that don't require sending all typing away to a third party.

> Your browser can do spellcheck...

Yes... But your browser won't detect "spellcheck do can browser your"

Grammar is more than just spelling.