That's because it doesn't (still) have a useful function. The "inexpensive" versions are crippled without 4G/LTE, while the 3G-enabled versions are very close in price to an MBA without a valid input method. I own a ton of apple gear, but just don't see the need for an iPad.
I think the problem here is that you're expressing your opinion without being clear you're expressing only
your singular opinion and experience, rather than making declarative statements of fact, particularly since millions of people apparently don't share your
opinion; you use loaded words such as "useful", "crippled" and "valid", but only the word "I" once, and only in relation to your possessions.
If 4G/LTE is important to you, then fine, iPad might not be the product for you. However there are plenty of people who are quite happy with 3G or Wi-Fi. Calling it "crippled" without 4G/LTE is like saying that any computer with less than 8 cores is crippled. Perhaps for you, but there's plenty of people who are quite happy on i5s, i3s... hell, the MBP I'm on now is a Core2Duo and it's fine by me. My mother is on ADSL 1 and she's happy. Her call phone is a monochrome Nokia with the Menu-* lock/unlock thing - her mobile internet speed is 0 bytes per year.
Likewise, I don't even own an iPad - I've used several at work and one of my friends' - but I find the input method completely valid. And enjoyable. If it's "invalid", it's only "invalid"
for you, rather than being a wholly inappropriate way of operating a device. And while you might not have found a "useful function", I have. More than once a work iPad has come home with me overnight.