Why do most rentals not include a washer and dryer?

They are definitely a pain. I think that the reasoning has mostly to do with cost. If the landlord owns the dryer than that is one more aspect of the rental the landlord needs to service.

In some cases they will provide a building laundry room and charge tenants for the use. (Another way to make extra money.)

It varies though. I've seen places that provide everything from washers and dryers to microwaves and free WiFi. Then again, I once lived in an apartment where they didn't even provide a refrigerator--had to buy a used one ourselves. So it all depends.

Good luck with the house/apt. hunt. Doing the same myself right now so I totally sympathize.