Why is it so expensive to send an email?

As others have mentioned, a lot of the cost comes down to managing IP reputation and deliverability. But there's also the infrastructure needed to send emails at scale, process billions of events (bounces, opens, clicks) and pass those events back to the caller, which all needs to happen within seconds at scale. Most users also expect click tracking, which means more infrastructure, more domains to protect against blocklists, and even more due diligence on who's sending through your platform.

That's not to say most providers aren't charging too much, IMO, but it's not quite as simple as it might first seem.

That said I haven't come across anyone charging $15 for 1,000 transactional emails, as the OP suggests. Probably a typo. For comparison, SES is $0.10 per 1,000.

Yes, there is actually a way to solve the end to end tacit permission problem, which involves cryptography (obviously). And yes, there is a way to make it ergonomic in a way that users could actually use and understand it well enough that the below-average user would do fine with it.

With this problem solved, all of the anti-spam stuff and all the trouble that comes with it becomes obsolete. Until this has happened, email is going to continue suck a lot.

Most of the work goes into reputation management and customer support and in my experience, most of the cost is marketing.

For raw sending there are plenty of cheaper options, as others have mentioned. SES, for example, charges ~$0.10 per 1,000.