Because it is a decision by committee. VAR room sh ould be there for offsides* and alerting the ref to incidents he needs to look at. Just give the power to the referee on the pitch as much as possible. Hell if you really didn't want to always stop the game for him to run over, make use of the fourth official for checking the monitor, at least he is there and saw it live. He can then get over the on-pitch ref in much needed cases.
*Adjust those offside rules.. not clear within 30 seconds, then the original on-field decision stands.
Cross into the box and Joe Gomez knocks it away with his hand
Ref misses or feels it is accidental
Liverpool clear and play continues
VAR are already checking and see that yes the ball hits Gomez's hand. Advise 4th official to check monitor
He checks and clearly sees Gomez move hand to ball to stop it reaching opposition attacker
Alerts ref - PENALTY - Games stopped
Luiz flies into a tackle but ref thinks he got the ball and game continues
VAR check incident and Luiz catches player above the ankles with studs showing, but does clip the ball
Advise 4th official to check, but he is unsure himself. So gets referee's attention who stops play.
Ref heads to monitor and decides it was a dangerous challenge and gives Luiz a red card.
In both cases the VAR room aren't making the decision, they are just checking the incident did or didn't happen and pass that information to the on pitch officials. Using the 4th official to check first and allow the match to carry on with little disruption until such a time it needs to be stopped (or stops naturally).
It all comes down to the ref that way, which everyone will accept.