Hypothesis: Once upon a time, the wizarding world had no popular sport of its own, and Quidditch was more akin to aerial dueling, a one-on-one contest of skill. Then, someone realised all the various benefits/opportunities offered by popular sports (perhaps by watching the Muggle world), and added extra rules and a team element to give the crowds something to watch while the Seekers continued their long periods of boredom interspersed with sharp bursts of activity.

Quidditch today generates a massive market in terms of matches, merchandise, contracts, celebrity culture etc. - a market that benefits the economy as a whole and certain key segments of it especially. It also serves various other purposes common to team sports, such as channeling the volatile energy of young people, and creating a harmless outlet for tension between countries (harmless in theory, anyway - we don't have riot statistics for the wizarding workl).

Whoever shaped Quidditch into its modern form didn't need a balanced game - they just needed something to fill the sport-shaped gap in wizard society. Such a hypothesis would explain why Quidditch is so poor in game design terms - unbalanced scoring, disproportionately high risk of injury and matches of unpredictable length don't matter quite so much if your goal is to pander to the audience rather than make a fair test of the competitors' skills.