I've done some comparisons looking at the specs on wikipedia and generally speaking tablets that have come out around the same times phones did, have cameras that are a few megapixels fewer than the ones on phones. Is there a technical reason why it's harder to have a camera in a tablet?
The manufacturer is trying to help you. Walking around taking photos with a tablet makes you look like a twerp, so they try to discourage you by not putting a good quality camera in there.
That might be a slightly silly answer, but it's not entirely untrue. Taking photos with your phone is really popular these days, because you always have your phone with you. The quality of the phone camera is therefore a big selling point, and manufacturers compete on that.
It's much less common to take photos with a tablet: not just because you look like a twit holding it up, but because people don't take a tablet with them on a night out, or a bike ride. Tablets are mainly for using at home on the couch, or in bed, at the office, or on a long train ride. The cameras on tablets are mainly targeted at scanning QR codes, and maybe doing a little video calling over the Internet. For this reason, the camera quality isn't such a selling point, and it makes more sense for manufacturers to spend the budget on other factors that consumers rate, such as the screen, the RAM, and the processing power.