We are tho. Because of the margin of error.
Arguably the system is not accurate enough to make calls on offsides where the measurement is showing a 1 or 2mm difference.
In those cases you might as well toss a coin. The system isn't capable of making such minute determinations with 100% accuracy.
Even the size of the pixels on the screen the operator is viewing could make a difference to the decision. He could place his marker incorrectly by 1 or 2 pixels, causing the system to change from offside to onside or vice versa.
That's why I say we're treating a 95% accurate system with the authority of an infallible system.
The operator never says, "I can't be sure of this decision. It's too close to call."
Because the operator never says this, the margin of error is not being taken into account. Thus the system is being treated as 100% accurate.
A linesman doesn't say that either though because 'I don't know' isn't an acceptable outcome, it has to be on or off. No 'it's too tight to call', on or off.
VAR absolutely has a margin of error but you're talking about something in the order of +-50mm at worst (probably less imo), which is still massively better than a linesman which is going to be more like +-50cm (or 5m if you're Man City playing Southampton
https://pbs.twimg.com/media/BkdZ4NNCMAEQBLq.jpg yes I'm still bitter about that goal).
Why would you want a system that might get it wrong by a few mm to defer to a system with a much bigger margin for error? "This system isn't good enough to judge 5mm so I'll use the one that's only good to the nearest half a metre instead, that'll be a more authorative accurate result". That doesn't make any sense.
We've got two systems available.
One is liable to make mistakes when you're down to mm, the other is liable to make mistakes in the cm to m range. Why wouldn't you prefer the one that's only prone to mm scale mistakes?
If VAR isn't accurate enough to satisfy you then a linesman definitely isn't accurate enough, so where does that leave you? There is no magic better system available today.