manual focus - Why don't DSLRs come equipped with "classic" focusing screens (split prism, etc)?

I have a hard time manual focusing my Canon EOS 350D.
Compared to the few higher spec DSLRs that I played with, and that have a pentaprism, my Rebel's (pentamirror) viewfinder feels crammed and somewhat darker.
I feel like I have to squint in order to figure out if the subject is in focus.
It also has a simple matte focusing screen, so it doesn't help figuring out when critical focus is achieved either.

I remember having used my dad's all-manual Praktica film camera in my teen years, and at the time I felt no burden with manual focus (exposure, now, that's a different story...)

I think the focusing screen on that camera was a split image + microprism setup, something like this:

Full viewfinder view: http://www.focusingscreen.com/picture/fsxb.jpg

Up close: http://www.focusingscreen.com/picture/fsxbs.jpg

All this being said, here's my question:

Why do (most of?) the DSLRs of today not use such focusing screens?
Do the manufacturers work under the assumption that most focusing is AF (which is probably true)? But still, how would it hurt to have a split image as a focusing aid?

Is there an obvious downside to such focusing screens, that I am missing?

For the sake of completeness, I have found two places that sell replacement focusing screens for DSLRs. I'm sure there are others.

This is the 1st one, I think based in Taiwan. They have a great comparison of different types of focusing screens (that's where the above image links go, too).

This is the other one.

The installation procedure looks a lot less scary than I would have guessed. I think I'll get me one of those, maybe it will put an end to my manual focusing misery.