Why dovetails?

Hi,

I'm new to this group and in fact new to CNC machining altogether. I
have some experience in wood- and metalworking, and am in the process
of converting a Proxxon MF70 micro mill to CNC.

In doing so, I'm dreaming of a larger mill already... I'm thinking of
building one from scratch, rather than converting a non-CNC mill, let
alone buying a CNC mill. Somehow I imagine that a design where the
workpiece is at rest (or on a rotary table) and the spindle moves
would be easiest and best. This would make the mill of my imagination
very similar to a CNC router, only somewhat stronger and stiffer.

There must obviously be something wrong with this idea since this is
not a design you normally come across among metalworking mills. Is it
because dovetails are so much better than any other kind of linear
movement? I was thinking of triangular rails with ball bearings loaded
against each side, would that be less stiff compared to a dovetail? I
don't need the stability of a Bridgeport because I have all the time
in the world and can take shallow cuts, but please tell me if my idea
is unsuitable even for my humble needs. It needs to be cheap in the
first place, but also capable of cutting metals, at least non-ferrous
ones.

Thanks a lot, and please excuse me if this has been discussed before.

András