The problem is how bullshit transactional emails are when you're outside of AWS. If you're not expecting to use 10,000 emails a month but would like the option to go over the free tier without committing to 10,000 more. Just let me pay per use FFS.
They could price per use, but it would have to start with a base fee that is about the same at 10,000 emails.
Also, the person who just wants to send a few 100 emails a month is actually far more likely to be a spammer. So it's also a way for them to eliminate those folks.
And lastly, the support burden can be high.
AWS has basically said they only want serious customers, let the other guys worry about the small senders.