1 They give people more freedom
So many people like to enjoy more freedom while playing games, not having to commit to finishing certain levels, or completing certain missions. Endless running games help those people satisfy that need.
2 They can be played for any period of time
Endless running games are perfect for people who take very short breaks because they can still play and enjoy the game without having to worry about completing a mission in their short break.
3 So many people aren't goal oriented
So many people aren't goal oriented, they prefer to move freely and do whatever they want without sticking to certain goals or objectives.
4 The game is played for enjoyment
Running games are usually very enjoyable. People play them to enjoy the game-play, and not to achieve a certain mission while feeling frustrated or unhappy. (See Why Online Games are addictive?)
5 Less stressful
When a game has no clear objective it becomes less stressful. The stress of having to achieve certain goals can be very intolerable to some people.
6 Being chased by something is very thrilling to many people
Games such as Temple run or subway surfers can be very thrilling because the fact that the character is being chased by something or someone triggers the fight or flight response.
7 Little or no competition
Not all people are fans of competitions. Endless running games have little or no competition and as a result make those who don't really like competitions more comfortable. (See Why are mobile games popular?)