Because Tamiya acrylics don’t thin well, if at all, with water. Tamiya’s acrylics are alcohol-based, not water-based. And even though some say they have used isopropyl successfully to thin Tamiya acrylics, that has not worked for me.
Through trial and error, I settled on using Tamiya’s proprietary acrylic thinner (X20A) when I work with their acrylic paints, and that solved the problems I previously had with the paint clumping, or coats lifting when I tried to apply successive coats. But their thinner is too expensive to use to fill a reservoir in a wet palette.
It’s not a problem for me, though, not to use Tamiya acrylics with a wet palette. When I use any Tamiya acrylic, I use a small jar of X20A and dip the brush in the paint and thinner, then apply it.