Why do computers do things we dont want them to?

Computers appear smart, but they're by no means sentient and are really just huge, complex machines that do what you tell them to.

If your computer is suddenly infested with adware and malware, that's not a mistake on the computer's part. It's downloading and displaying what it's been told to, and the 'wares are just doing what they've been told to do, as well. If your antivirus missed something, that's not so much an actual problem with the computer, as it is just something that humans have missed or not known to 'block/delete/quarantine.'

Some things do appear to be random, though, like data corruption/files randomly disappearing/etc. This usually happens because of physical damage to your hard drive, or 'soft' damage that's been caused by 'wares or incompatibilities. Physical damage is either unavoidable or human error, while 'soft' damage is usually either malicious or because something unexpected has happened that your computer doesn't know how to react to.

before a program is allowed to access your hard drive at all, shouldnt it have to ask you the first time?

By just using your computer you're already reading and writing hundreds of different things within your hard drive. You couldn't possibly whitelist every single change that's made by yourself, which is why computer manufacturers/software engineers/etc rely on their customers to not make mistakes.

There are some things that you have to allow, though... so it does ask you already? You probably won't be asked if you're trying to open something by Adobe, a huge and trusted company, but software made by smaller companies often isn't recognized instantly as 'safe' and you'll probably need to allow it to make changes as an administrator.

Before a browser plugin acts on other plugins to change how they work, shouldnt it have to ask you the first time?

It does. You can even be a stickler and tell your browser to ask for permission before running javascript/flash, and before opening popups/new windows.

shouldnt we be able to uninstall all the effects of those bad programs? Why do computers do things we dont want them to do and worse, then refuse to go back to the way things were?

Same answer for both of these; you can. Like I said, you can't make a backup after every single change that you make, but your computer will often back itself up before installing big things like system updates, things that are known to sometimes run into issues. Most OS' will also let you make a backup whenever you want, that you can either save on your hard drive or upload to a disc/USB.

TL;DR: A lot of mistakes are really just human error somewhere, or because something unexpected has happened. Depending on which type you have, your hard drive can also ware itself down or straight up destroy itself if you move it around too much. All in all? Your computer already has tons, and tons of fail-safes and backup options for you, but common sense is still the easiest way to prevent this sort of thing.