Apple doesn't need to invest engineering and development resources to develop a $50 AppleTV anymore than Porsche need to develop a $20,000 911. What profit would Apple make? and it would cannibalise sales from the ATV 4K. (Where I'm guessing they make a very nice profit, why build 15 $50 ATVs to make the same profit you can from 1 ATV4K sale?). It's about profit not marketshare.
Google can do that for them and that way apple make money on ATV+ subscriptions from people who would never have paid for an ATV in the first place.