In layman's terms, an inboard engine in a planing hull works very hard

We have a winner!, add to that, we don't have 140+ MPH air going through the
radiator like you would have with a 454 Corvette at WOT and you start to see
the problem..
Water is such a good coolant, it would be silly to ignore it, if you are
sitting in it. That is why they like to build power plants by the river too.
The real trick is to heat exchange the "nasty" water without actually letting
it get into your engine (which runs glycol mix). In the case of an outboard
that is not worth the weight and money. They just try to harden the block with
aluminum and zincs.