It's also entirely possible you're looking at homes that aren't for sale. If you think about this from the owner's perspective, having your interior photos and a floor-plan of your house online for anyone to see at any time isn't a great thing from a security standpoint.
You will rarely see a floor-plan even if the house is actively for sale. Few owners want to pay to have them made. The best you can hope for is for the listing agent to take the photos in a way that lets you piece together the layout of the house.
Something else you can do is work with an experienced buyer's agent. Particularly if you're looking older housing stock, agents have often seen enough of certain types of houses to be able to give you a fair idea of what it's like inside.