According to which definition please? You have just outlined the general definition for a computer above and the iPad is clearly one.
I agree, iPad is a computer. It exists in a different paradigm of computing, but nevertheless it is a computer. It has applications, web browser, ability to play media, communications, etc.... I don't understand why people claim it's not a computer.
That being said, I've tried multiple times to use my iPad as my main machine, but have failed to find a workflow that works for me. I find that managing files is much easier on a laptop/desktop then the iPad. I use iCloud so it's possible, but proves difficult. For example, when I open a PDF through Safari and try to save it to iCloud it doesn't give me the ability to rename the file before saving it. I have to save it first, then go into the iCloud Drive app and rename the file from there. Not very efficient, but doable. I guess I don't have the patience for it haha.