The law used to be that level was offside. Before it had to be 3 defenders, not 2. The law was also changed so that an attacker had to be interfering with play to be offside. It has changed several times in favour of the attacking player.
Yet we see situations regularly where the lino puts his flag up because he is "obviously" offside and VAR then shows the lino got it wrong. Offside is a factual decision based on the relative positions of the attacker, the ball and 2 defenders at a point in time as defined in the offside law...
It took a long time but I expect that was because they had to check the defender on the far side as well as the ball position, and they have to get the best frame to represent time of ball contact. Some mistakes have been made with offside decisions so they probably want to be extra careful.