Treadmills are major pieces of equipment and to buy a decent one, which is what you need, is not cheap. To buy one that will last and has the features you want you will be spending around $1,500+. Then you need the mat to go with it and regular maintenance. You will need to pay for the lubrication and other sundries that it needs.
The cost doesn't stop there, once you get it delivered you have to assemble it. Assembling treadmills is difficult and takes time. It sometimes takes 2 people to bring the treadmill in the house and you will need 2 people to assemble it. Unless of course you pay to get it put in the right room and assembled for you (for a price).
What you could do is join a health club or gym and use their treadmill. If it was just a treadmill you were paying for it wouldn't be so expensive but they come with all their facilities and staff which all costs money. This money is passed onto you the customer to pay for. A lot of those facilities you may not ever use but you'll still be paying for them.
The space they take up
Treadmills are large pieces of equipment and you need a large room or garage to put it in. You could try one of the fold up treadmills but the folding mechanism is another thing to go wrong. They tend not to last as long and even when folded can take up a lot of space. The decent treadmills are the ones that don't fold.
The large room will probably have little more space apart from the big treadmill in there. Maybe you have a basement or home office space you don't use. Maybe you have a spare bedroom you use. Either way the only use that room has now is the treadmill. Some people just don't have a house where they can put a treadmill in so have to go through the expensive gym route.
They need professional maintenance and repairs
There is some maintenance you can maybe manage yourself and that's the regular lubrication the treadmill requires. You need a special treadmill lubricant for that. For the most part any maintenance will need to be done professionally. The calibration of the belt and the cleaning of the motor and rollers. These will take time.
You have to be indoors when the maintenance man comes round and that could be difficult if you go to work for a living. Not to mention the cost and you thought you would only be paying for the treadmill itself. Hopefully it's still under warranty so it doesn't cost you any money, just the time and hassle of being there.
The loud noise
Where ever you put the treadmill know that it makes a lot of noise. That's the treadmill itself and you running on it. Someone in the next room with a television on is going to be disturbed by it. Let's hope it doesn't start squeaking because that could get very annoying. If it's one floor up then whoever is downstairs will hear and feel a pounding on the ceiling.
If you live in a first floor apartment or apartment block they may not allow you to have a treadmill because of the noise. A mat will mitigate some of the noise but no matter what the mat makers will claim, some of it always get's through. I'm sure there are even cases of a loud treadmill being cited in divorce papers due to the problem!
The sheer boredom
Many will say this should be first on the list. Running there on the spot pounding miles looking at the same place can be boring. Maybe your treadmill is in the garage or you haven't set up a TV yet. Then you're fed up with looking at the spot on the wall in front of you. You can only look at it for so long before madness takes over.
The lack of nature and the lack of other people can also make it get on your nerves. The scenery is no different from one minute to the next. There are no birds whistling their beautiful song. Just a repetitive pounding of your feet on the running surface. You can't run in any different location and appreciate anything new you see, it's just the same old run as always.
They don't replicate outside running
Running outside is very good for your muscles compared to running on a treadmill. It's very different because your lower leg muscles get used to supporting each leg as you pass different terrain. The terrain doesn't change on a treadmill and your muscles suffer for it. You may get overuse injuries from running on a treadmill.
If you're about to run or train for a race then practising on a treadmill does not compare to running outside. You don't get any wind resistance and the belt moving beneath your feet is certainly something nature can't replicate. If you're training for a race you need to ideally complete a certain number of weekly runs on the same surface and the treadmill doesn't cut it.
You look at all these disadvantages of using a treadmill and you start to agree (I do). But then you think that 50 million people in the US alone will use a treadmill at least once in a year. The sales of treadmills keep many fitness equipment companies in business. Gyms and health clubs are springing up everywhere.
There must be some very good reasons why people are choosing a treadmill despite of what's written above. For all their downfalls they are convenient and safer than running outside. Although people do get injured from treadmills but few compared to the dangers of running outside. So it's down to the individual person, some like running on treadmills and some don't.