Because that pairing wouldn't work. I can see your logic but one is a lad without much experience who plays off the cuff and the other is a lad who looks scared of a football at the minute.
I'd be pairing Holgate with Jags until the end of the season, telling Keane to go away and sort his head out and make sure he's ready to come in pre season ready for a fresh challenge and to start his Everton career afresh. Knowing he won't have to see Ashley Williams ever again.
If his head is really gone, that's fair enough. I'd rather clear his head for maybe 2-3 weeks and give it a go. Playing next to someone with Holgate's range could give Keane more confidence and allow him to play to his strengths. And then if he still sucks, you know there is a permanent problem you have to fix, and/or have an open competition with Funes Mori for his spot next season.
I suspect we will have to use the bulk of our funds on LB and a creative passing midfielder this summer.