There are 3 aspects to consider:
1) Everybody needs around 3 days to get the feeling of a place, new site, new guides, different weather. With a week you start to feel comfortable just before leaving, with 10 days, you got a week in front of you!
2) Even in the best spot in the world you can have some non-flying days, on a longer stay it doesn’t matter anymore. It gives you some time to go kayaking, visit the ancient theatre, it’s more like holidays!
3) Most importantly, pilots are more relaxed, not in a rush to fly all the time, and that improves safety a lot. It’s also in these last days, that the average pilots or beginners see the results of their progress and make their best flights.