Why on top of SES? why not SMTP?
Because delivery rates through self-hosted SMTP are lower than through commercial e-mail delivery services; the latter being close to unity[1] and the former ranging from near-zero to about 7/8 depending on how much work and time you put into doing things. 1: My "close to unity" number comes from my experience with the mailgun APIs, I assume that SES is similar.
Imagine the outrage if your national snail mail carrier were to shred 50% of all mail which didn't have a Premium Protection stamp. Why do we let the big cloud hosters get away with it?