Why is a new website sandboxed by search engines?

Search engines consider a new website doesn’t have high potential to get high traffic and especially if it is achieved through SEO. They sandbox new websites for few months so that only old reputed websites may appear in top 10 after they are optimized. These help visitors in getting most relevant results from search engines. However new websites can attain high rankings with slow and regular SEO for their website. Due to sandboxing of new websites, any new website can loose traffic quickly or old websites may get higher priority for achieving high rankings.