built-in phono stages -- good idea?

The main problem with this is you are talking about the most difficult to design component in all of high end audio. The phono stage has to take the weakest signal in all of audio, amplify it by the most gain in all of audio, and equalize it with the wildest and yet most precise EQ in all of audio. Any one of these is a monster challenge in and of itself. The phono stage has to handle all three!

The more you look into what goes into a good phono stage- and turntable- the more you start to appreciate why they aren't very often combined into one. Noise rejection and quality power supplies are critical. Turntables have a motor, which you notice they always put as far away from the cartridge and arm as possible. Because of the noise. So now you would add an additional power supply and phono circuit in there, and have to shield all that from the motor as well.

Power and shielding are way easier (read, can be done more cost-effectively) inside a pre-amp or integrated amp. 

The main market for tables with built-in phono stages is entry level, because there you have people looking for plug and play, and this makes it simple. 

Emt tables are built in. I think Walker Audio tables  almost do this  by connecting the arm wire directly to the phono amp. It’s just not convenient enough to be viable option. Even the best design isn’t necessarily easy to market.  

The power supply would be external of course. Only low voltage in the turntable + shield box on the actual compact phono stage.

I don't know... it seems like a good idea to have the amplification as close as possible to the delicate signal source, to reduce the amount of noise being picked up. It is now possible to have compact, high performance phono pre amps thanks to SMD.and good opamps. Just seems logical so I must be missing something ???

The Naim Prefix is one example of a commercial product that is based on those principles.

There is nothing stopping one for placing the phono amp right behind or below the table/arm/cartridge, with an uninterrupted run of internal arm wire from cartridge tags to RCA plugs going right into the phono amp jacks. That's how I and others do it. If you can get the phono amp close enough to the arm, the wire need be only 1/2 meter long.