Because that's the way we do things in this country, there is a limit & it's a one size fits all approach, across a multitude of life, age of consent, smoking, voting, speed limits, bowel cancer tests, etc. Your friends at whatever age could well be far better drivers than those in their 20's but that's not how it's done. The DSA test people to the minimum standard in a hope that over the years they use their experiences & become better drivers, I believe/know there are people who peak at their driving skills on the day they take their test & go downhill from there.