But give them some credit. Most of these problems are solvable. You can build a drone with bumpers around the propellers and screens above and below, to solve the "sticking finger in the propellers" problem.
You can deal with the vast majority of the navigation problem by just getting enough altitude. Above a certain altitude, you don't suddenly see new unknown buildings springing up out of nowhere, and don't need to worry about navigating around power lines or anything of the sort. Just do the takeoff in a controlled area, rise up to an altitude high enough to avoid anything but a few pre-programmed skyscrapers and radio towers, and fly to your destination at that altitude.
And you can solve the landing problem by having someone sit in a control room and take over for the landing and takeoff from customer locations. Paying someone for two minutes worth of landing and takeoff is much cheaper than paying them for the full 30 minute delivery time, along with the expense of a full sized vehicle and insurance that goes along with it.
Everyone seems to worry about the liability aspect, asking "what happens when one of these falls on someone". But we already deal with liability issues with cars all the time. Heck, we deal with the issue with planes as well; planes do sometimes fall out of the sky and kill people, but you don't hear people saying that delivery by plane will never work. That's just built into the cost of delivery. Drones are much lighter, don't have tanks of gasoline, and flying in the air gives you much more open space than driving on roads. And you could probably add a failsafe that deploys a parachute if the drone loses power or otherwise start to fall.
The big problem is flight time. Batteries just don't have the energy density to support long flight times; you can build ultra-light drones that get an hour of flight time with a lot of care, but if you're building something that's actually going to carry packages, deal with wind, and so on, 30 minutes is probably the upper limit. That means that you won't be able do trips of more than about 15 minutes each way. Now, since you can go in a straight line, without traffic, that gets you further than a 15 minute car/bike delivery, but it'll still be limited to people who are fairly close to a distribution center.