Again, if you don't like the poll, start your own.
This is a comparison to the Fighter poll, which uses exactly the same language.
The point is, it's showing that while in higher level play, there is a very large minority of groups where single classed, non-magical fighters are not played at all, there are almost no groups that have no single classed magical characters. Additionally, it's pretty fair to say that many groups are seeing multiple single classed casters at high level.
So, the question is, why? Why the discrepancy? If fighters, as the most popularly played class in the game, have the majority of tables with no or only one single classed fighter at higher levels, why? Casters don't have this. Almost no tables, less than 1 in 10, are seeing zero single classed casters.
The point you are missing here is the SINGLE CLASS element. I'm sure that fighters are pretty regularly played. That cannot be denied. But, I suspect that in higher level play, most of the fighters you see are multiclassed into some sort of caster.