Since it is E-ink, is there a way to keep the last page read on screen instead of screensaver/ad?
(And if not, why not? it is a killer feature...)
You're right, there is no reason for it not being technically possible. In other words, you can do that if the firmware lets you do that. Which, AFAIK, the native Kindle firmware does not. That leaves you with either a hack or jailbreak as your options, if you're feeling adventurous: I found a relevant thread on Reddit that could point you in the right direction. (Also, I have been educated it's called "off screen" not "screensaver", as it's not saving anything. Which... I don't agree with, see below.)
As for "why doesn't Kindle allow this", that's an interesting question.
I would theorize that it's for the same reason we used screensavers on CRT monitors: replacing a page of text with a picture helps to avoid ghosting/residual image and therefore extend the life of the screen.
I mean, aside from the obvious, which is "we gotta put the ads somewhere" ;)