For the same reason you don't want to use pressure treated lumber for the bunks on your trailer.
The chemicals used to treat the wood tends to react with aluminum, causing premature and/or advanced corrosion where there's contact.
It's a somewhat long-term process, but there's been a number of disappointed folks that found pinhole leaks right where thier pressure treated bunks contact the hull even through old carpet. Also, for those that have replaced transom wood with pressure treated, they find further down the road, that they're having to replace entire transoms due to excessive corrosion.
Roger