I think they would have something to lose by admiting it which is why they don't. If Google admits publically that new sites (however great the content) don't rank well for up to 6 months or even a year, other engines, especially MSN can market theirs as the most up-to-date SERPs. Search is 50% buzz and Google could tarnish its reputation.
The existance of the sandbox makes perfect sense and is a major deterent for spam, but Google, however much they try, shouldn't be allowed to have it both ways... boast SERPs with less outright spam vis-a-vis other engines (for which the sandbox is a major aid) while also pretending that the sandbox doesn't exist and that their SERPs are are as up-to-date as the others (which they are not). For new sites, MSN ranks them far earlier than the others (but also pays a price for doing so).