Yes but with mails, you read... mails. That is what you consume. With readers, you consume content that is most probably there because of potential ad revenue. Yes, RSS is generally used by technical people - but you'd be surprised how many technical people don't use adblocks, sometimes because they don't care and sometimes because they don't think it is ethical to do so. And there always is the possibility that it can evolve into something that is more convenient used by masses. So it is a net negative in terms of revenue.

And I'm not only talking about people that produce readers, but also about content creators. I'm making a site and hoping to turn a profit through ads and my interesting content - why would I want to provide a convenient stripped down version of my website for you to consume - something that is a net negative for my revenue (hosting and transferring data costs money)? Sure, I can provide "excerpts" in RSS, but I'd rather prefer you go to my homepage to see what is new - it is thoroughly AB tested to lure you in, spend some time and see some ads - maybe click one.