In a pre-drone world you can get explosives, divide them into X equal sized packages, add a timer set to the same point in time to each, then travel around the country hiding them in high-traffic areas.
Yes, that approach is inferior to the drone version. You have to hide them inconspicuously, and a bomb sniffing dog could find them. But you can visit a lot of places in a single European country or US state within one day, and unless the country is already on high alert you can hide something for that time span in public. Yet this doesn't happen. Even regular bombings are rare.
The reasons are manifold: In most places getting explosives isn't actually all that easy (unless you go the homemade route) and is a good method to get attention from authorities. But another factor is that there just doesn't seem to be a large interest in doing that kind of complex attack unless there is already an ongoing civil war. Actual terrorism is fairly rare, and the terrorists tend to be not all that sophisticated.
Are these kinds of drone attacks a scary new possibility? Yes, absolutely! Are they likely to happen? Not really. We might see it as a method to assassinate officials (imagine staging drones at a place where you know the US President will hold a speech in a couple months), but I doubt it will play a major role against the general population