Why do TV networks care more about having an unrealistic cast of minorities more than their ratings ?

It's to promulgate the liberal view of the world which you must subscribe to. The broadcasters are their propaganda devices. You must have realised that?

So many US programs are filmed in Canada nowadays it's been nicknamed 'Hollywood North'.

Those aren't unrealistic portrayals in Canada.

If you've got a problem with it, well, the US must be more backwards & less civilized than anyone thought.

It's very misleading to us foreigners. I grew up watching American TV shows full of multicultural casts and assumed that America was a mixed culture just like the rest of the modern world.

The first time I visited America it was deeply shocking to see a non-TV version, and just how deeply racist and segregated the country is. Even people I knew and stayed with and who were by no means intentionally racist only mixed with other white folk.

I guess TV shows do it because maybe they're hoping that by showing mixed-culture images they might in some way bring American society into the 21st (or even 20th) century.

You mean the all-white characters on Gilligan's Island, Friends, The Beverly Hillbillies, and The Brady Bunch were real?

If so, they made white people look mighty stupid!