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.