Why can't people just reply an email? Is it too hard?

Not everyone has permanent access to the internet or enough time to be replying to each email received. If you think your message is important enough, you should try using the phone.

Edit: I forgot to add something, not everyone gives the same level of importance to emails. As I mentioned before, for some people, an email is a low priority message, since many would expect a phone call for something important. I guess it really depends on your type of job.
This also happens to text messaging, Facebook and other social networks. In my case, FB is not really important and if you send me a public message, I might wait for more than one week to reply. For some people, that is like an insult or disrespect.

Some people simply aren't good with emails or they're busy or whatever else they want to tell you.

For example several of my closest friends have simply stopped replying on a regular basis. We used to email at least 3 times a week and now when I write them, I am lucky to get a response in a month!

I think some people just expect you to wait. I know that my former employer's point of view is that if you don't want to wait for a response, then you weren't that serious about your inquiry to begin with. This notion really irritates me because it's like saying you're important and that everyone else should literally just wait until you FEEL LIKE responding. Also, some people just don't care. Others really just suck at it. Etc.

I, like you, find it really rude.

Unless you are a real friend or someone at my work, give a reason why I should be obligated to respond to your e-mail? E-mail makes it very easy for strangers to waste a lot of time for other people who have to read the message and then reply.

maybe that person gets 500 emails a day or just doesn't check their email very often, or their computer is not working, or they are busy and nowhere near their computer

if it's that important - call them