So it's the margins.
The concept is easy, offside is binary, you're either onside or you're offside, so every decision is properly checked in relation to clear and obvious. They have specific guidelines which they follow in order, as accurately as the technology allows them, to determine if the decision is correct.
This means that there's a system in place that should result in teams being treated the same way, which is good. Sometimes a referee will still feck up, which is bad, but it's not something that frequently happens.
Now, if you introduce something like "human eye", then all you do is ensure more differential treatment. Some referees will instantly look at a situation and go "oh that's too close to tell, so feck it" and some referees will bring out the lines, then you'll end up with two close call situations in the same match where one, due to the angles, appears to be more onside/offside than the other so they are treated differently by the VAR even though they are just as marginal, then it'll turn out that one team had scored a goal that was offside while your onside goal got disallowed and the shitshow continues.
What will happen is that technology will continue to evolve, we'll get a semi-automated system and then higher accuracy and it'll overall be an improvement.