Too much debris and dropped oil in the middle of the roads. I typically stay in the RH "track" of the land I'm in ... that gives you even more room away from the on coming lane. If there is nobody coming down the opposite lane I might ride in the LH "track" of the lane, but will move over to the RH track when cars are coming towards me.
On a wet road the centre dries first, so will move there rather than ride in a wet wheel track. I will normally sit in the outside wheel track so the driver can see me in their mirror, they check that mirror more often than centre or passenger mirrors.