the more work i do with tyres, the more I'm convinced that the tread pattern is lttle more than fashion.
the fact the tyres with opposing tread patterns proves exactly that imo, bike tyres need "a tread" in the wet, the pattern makes bog all odds. Full wets have a heavy tread form two reasons, 1, to shift water, 2, to allow the tread blocks to move and generate heat making the compound more sticky, which is primarily what makes wets so grippy in the wet, not the tread itself. The second reason has bog all to do with road tyres as they would melt in dry conditions.
The percentage of tyre/tread or gap in the contact patch may come into it, but only to make the tyre more suited to wet or dry conditions, track day tyres and winter tyres for example? The shape of the grooves? makes no odds it seems to me.
So yes pure fashion imo. Tyres manufacturers making the most of a legal requirement. And why not?