People like Carl Sagan were pretty confident that there are aliens out there in the universe. He's still has a reputation as a great rationalist.

The fact that you would personally Occam's Razor away reincarnation given what you know, doesn't mean that it's also rational for other people to Occam's Razor it away. Someone who remembers a past life and who knows other people who do it can be common sense to have a prior that reincarnation exists.

If you start with a piror that reincarnation exists I don't see the scientific evidence that suggest that you should drop the belief. Assuming that reincarnation is true makes some things that involve working with memories of past lifes easier. Occam's Razor is all about making things easier.

On a practical side there are scientists who believe that reincarnation into Boltzmann brains is plausible, given their current models of how the world works.
If you belief that there are random fluctuation in vaccum, time doesn't end somewhere in the future and that the existance of humans is completely accidental it's hard to avoid the conclusion that reincarnation will happen.

If you think you "understand" science than you aren't rational. Any good skeptic should belief that he doesn't understand it. Human have the habit of being much to confident in the beliefs that they hold. The Black Swan by Nassim Taleb is a great book.