Quite simply they are designed to be addictive. There are literally millions of different games available. Video game designers have to make their game popular. Popularity occurs when players return to play on a regular basis. Designers achieve this by making their game challenging enough that very few people get to the finish. They also ensuring that there are incentives or wins along the way. We call this intermittent reinforcement. It fires up the pleasure areas of our brains and is a great way to ensure a person wants to return. This is the same type of reinforcement that makes gambling so addictive.
There has been a big push in recent years to increase the availability of computers and tablets in schools. Now many primary schools in Australia are introducing ‘bring your own device’ policies for younger and younger kids.