In fact, you just need to leave a backpack with a pressure cooker with some nails and explosives like in Boston. No need for complicated technology and millions invested to try and counter this.
Today or in the near future, plant a bomb anywhere in public in a city and you're likely to end up on camera.
If a terrorist can accomplish the same outcome while not killing their self (suicide), they get to keep killing; it net lowers the cost of terrorism dramatically. The biggest cost in suicide bombings directed at the developed world, is getting the bomb carrying person into location (immigration, housing), feeding the person, and the time it takes to arrange it all. Drones mean you bring one person in, and they get to take action potentially numerous times.
Shakers in the 18th century made some of (in my opinion) the most beautiful wooden furniture with simple hand tools. But each piece took ages to produce and finish.
Now we have power tools and inexpensive stains and finishing products that dry quickly. The output of a legion of Shaker craftsmen can be matched by a single moderately skilled woodworker today.
Similarly, the fact that attacks were successful without drones doesn't mean that cheap, commonly available drones can't make it easier to do lots of damage. Nobody says you need these items to make a successful attack, just that they have the potential to make a bad attack much worse. Not to consider this while implementing countermeasures is irresponsible at best.
Leaving the bags is arguably easier to do.
These are by no means significant deterrents but a drone is free to go anywhere, anytime with right now nothing to stop them.