Yeah but that doesnt suit those that want to freeload off someone elses Sky subscription. Or want it in another room without having to pay the multi-room price. So its just not convenient.
It can also be due to content license agreements. Yes the agreements can be "you can only let your customers play this on a phone/tablet and not chrome cast / apply play it to a tv" As Android and iOS is an all or nothing type of thing with casting then there is not much sky can do and to fall inside the agreement they might just need to kill it for all media within the skygo app
Content agreements can be very interesting. You might be able to stream it on your tablet in your home but not on the same table over mobile data etc.
Kinda like the NRL on Telstra in Australia. Limited to a small screen - as someone else has the rights to the bigger screen. I'm sure that Telstra wont let the content be cast for fear of infringing on others NRL rights.