Why do agents fail to e-mail rejections?

I've been querying agents for some time and have lived through the change from snail-mail to e-mail. In the 'old days' a snail-mail query was always accompanied by the requested SASE. The vast majority of agents always used this, even though it was only to send a form rejection. Today most agents accept e-mail queries. In fact, many will only accept queries electronically submitted. Why now do many agents state that they will only respond to a query of they are interested? I understand agents are swamped with queries and would rather spend their time working with clients. Even so, I feel that the lack of a simple e-mail form rejection is an indication that an agent has less an less regard for potential authors. In fact, since it appears to be frowned on to send an e-mail query with a read receipt request, the prospective author has no way to even confirm their query was received. Based on this situation, I have seriously contemplated only querying those agents that accept snail-mail so I can send the old SASE and expect to receive some response, even if it is a form rejection. I must say that a few agents do send form e-mail rejections. Some even send personalized rejections. And a rarefied group even confirm receipt. Hooray for those caring agents.