As that page makes clear, SES can hand off incoming mail to a Lambda, or to S3 – or to SNS which can deliver it to any HTTP endpoint, or e-mail address, or text it to your phone for that matter.

What do you expect SES to do with your mail after receiving it? S3 and Lambda are optional delivery locations, amongst other choices.

Exactly this. It receives the email, now what? You need to run some code on it and so the way to do that is one of the compute services. AWS isn't forcing you to do anything here.

99.9% of SES users I promise are only sending mail anyway. You aren't forced to have Lambdas or anything else to send mail.