Why can't apps send an HTML email to the user's email app?

Sending html emails from apps has always been a pain in the ass. You either have to pay a 3rd party service to do it and implement their API or build your own email server and neither of those are great options for either the developer or the user.

In my case, I have an invoicing app and this issue has been a pain since before we even called a web app a "web app".

We can create an html link to send a plain text email to the user's email app. For my apps and my users it would be much easier and better for them and me if they could click a link and send their html invoices this way.

I get that malicious links could be inserted in a html email by an app this way but you can do that with plain text too so it seems to me that the user could be allowed (required) to approve apps in their email app preferences to allow them to do this as opposed to not allowing it at all.

After all these years, and it's been close to 20 years for me and my app's users, sending html emails from an app should be much easier for both of us and the best way I can think of is allowing them to click a link to send it with their email app, just like they can with plain text email now.

Since we still can't, I expect I'm missing why.