My guess the trend for older sailors to go to powerboats is:
degeneration of muscle mass .... loss of strength as one naturally
ages.

Its far easier to pull/push a throttle than to wrestle with a
flogging/sodden/angry sail on a pitching deck. Most of the 'oldsters'
that I know especiallly those who bought 'large' sailboats can no
longer muster the strength to handle them well.