Because we have tried and failed.
Our children share a room. The eldest bought an Xbox with his birthday money a couple of years ago.
We have room for one TV in their room and no more. The youngest loves it, wants to have a go, wants his own, wants to share.
The eldest, give him his due is very generous but like most people has his limits.
Ordinarily we'd tell the youngest to save for his own and this would work but this is a gaming thing and I don't know what to do.