I think what often happens is we learn to windsurf as teenagers and get used to the idea of a small board – with the volume that suited us back then. And then we typically put on weight as we age and are reluctant to increase the volume of our favourite kit.
Plus it depends where you usually sail, in that you may choose a board which is the right size for your home break, but when you take the same board on holiday you may find the rippy/white water conditions require more float than you have.
There is also a historical thing where people were told that 'sinkers' were the thing to have.
I'd say the advent of multfin boards is changing all this – with shorter fins that don't overpower, people are now discovering that larger volume wave boards don't over-power so readily in strong winds and so have a broader wind and sail size range.
A final point is that some prefer to sail high rocker wave boards that don't plane so early – i.e. they prefer to blob out and then catch a wave on the way in which is then ripped apart with the looseness of the board they have.
This is the opposite approach from sailing a fast tail or FSW board – where you blast out at speed but then find the board rocker too stiff to really shred a wave face on the way back in.