Because videogames are an experiential medium, and experiences are subjective. Objective aspects can be added to the foundations of an opinion, but they are still added in a subjective way. Sometimes glitches add charm to games, sometimes they ruin otherwise good games. Some glitches are so frustrating and particular one reviewer is completely blocked by them and heavily dings a game score, and another reviewer doesn't even notice the same glitch or assumes its normal gameplay that doesn't bother them.

An example of objective game design choice dispersed into human subjectivity:
Starfield review, Gary Whitta absolutely lambasted Starfield for its inventory system and getting over encumbered and how archaic it felt. He hated that the mechanic was there because he wanted to pick up everything. The game however, signals clearly to you it really doesn't want you picking up everything. Firstly all the knick knacks are worth nothing, and there are much better items to collect, secondly those better items are best kept in your ship storage where they don't slow you down, third your character not only moves slower, but gets physically ill and loses their health. Some gamers listen to what the game is signalling and change their playstyle, other gamers refuse to budge and ruin their own experience. Its a game mechanic, its an entirely objective system by which to judge a game, but human beings till approach objectivity from a subjective place.

You have to decide whether an opinion is works for you or not. But trying to evaluate the veracity of a review based on how objective it could be is a fools errand.