Feel free to pass them by. There are lots of reasons. And some of them are legitimate.
In some cases, the sellers are concerned about their privacy. Maybe they've had break-ins. Maybe they've got some valuable pieces of art or collectibles. Maybe they don't want people driving up, tromping through their front yard and back yard and peering into windows. Maybe a female seller has been through a nasty divorce and doesn't want to give her ex any help in tracking her down.
In other cases, the place is a dump and would only be of interest to investors or rehabbers. Even then, it makes sense to post pictures. But lots of retail agents don't, and sometimes for good reasons. It may surprise you, but if you post a picture of a dump you'll get lots of calls from folks who say: "Why are you selling that piece of $!##?" Or "I'll give you a dollar for it." Or "So, what are the sellers going to do to fix the place up?" The point is: It's being sold as-is. That's what the seller wants and that's what the agent agreed to. But lots of retail buyers expect to see nice, pretty pictures. And--inexplicably--they're offended if they don't like the pictures. The agent and the seller both know that a serious buyer will read the description, understand the type of sale it is, and--if interested--go out to tour the property.
And, frankly, there's no way to satisfy people who demand pictures. Understand: I agree with you that the most effective listing will have exterior shots of the property from different angles as well as interior shots of the main rooms. But one time I had a property and I'd taken plenty of shots, exterior and interior. But--oops--I hadn't taken shots from the inside looking out the windows to the outside. There were outside shots to show what was surrounding the property. But some supposed (not real) buyers complained because I hadn't taken shots from the living room and kitchen looking through the windows so that buyers could see precisely what the view would be from the windows.
So, if there aren't enough photos to satisfy you, pass the properties over. In some cases (as with privacy issues), that may be your loss. In other cases (as with some rehabs) you weren't going to consider the property anyway.
Hope that helps.