I think it's because the price they need to pay for putting an ads on TV is so high and also enables them to be seen my millions of people right away, so regardless of the quality of their products they still do so in order to race with other advertisers.
I think a few things factor into it, really. Low Quality - Most of these products aren't made to last. If they work correctly, they might hold up for a year or two. Most people end up overpaying on these items and they're not all what was expected when they purchased it. Unreasonable Shipping - Good majority of the TV ads advertise "Get 2 for the price of 1! Just pay shipping!" and shipping will end up costing more than what one of the items does by itself. Some people don't notice this when they check out, causing them to get an outrageous price for whatever they ordered when it shows up on their billing statement. Bad Customer Service - Ever tried to return one of these products? It's super easy to return it, good luck getting your money back, though. They'll take the item, usually you have to pay for shipping back, but you'll never seen your money.
Because of false advertising on TV. Most of this "As Seen on TV" things offer more promises than actual results. I bought one of those tile cleaning formulas once and the price was over the roof but I have high hopes that it would clean my bathroom, but after hours upon hours of scrubbing nothing happened.
A few people have already mentioned low quality and that was what we experienced when actually buying something from a TV advert. The TVB channel kept hammering out the "Microshaver" advert at every commercial break a while ago which eventually prompted my uncle to ask me to purchase one for him; we got it and it eventually fell apart within a few months.
Most of the products are nothing more than cheap junk, which is why people give bad reviews, for the amount of money they are spending they are expecting something of more quality.
As seen on TV products tend to never live up to customers expectations. This is because the advertisement make a product seen like it does more than a truly does. For example, I've bought a blender that was seen on tv that promised to be a lifetime product. After only a month of use it was broken. This is one of the reasons why I enjoy shopping inside of a store so that I can see the actual quality of a product. In my opinion. companies should make sure that their advertising matches what a product is said to do.
