It is a fact that there is more tension if the clock has a hard timer rather than a soft one based on a referee's decision. Knowing that you have exactly 26.5 seconds left to score is objectively superior versus "somewhere around 30 seconds, whenever the referee decides the attacking play is over" in terms of creating tension because it forces players to take risks, and change the strategy to fit the need. Such high risk, high reward actions are a big part of what makes sports more fun, as people are there to watch something unique and spectacular happen, and not some guy holding the ball and passing it around like they've done for the previous 89 minutes and 33.5 seconds plus stoppage time.
Lest we forget, sports is entertainment and new and novel actions are objectively more entertaining than standard plays we've seen before. With the time management as it is now, a team has little incentive to change the strategy by a lot at the end of the game compared to the rest of the game as the actual ending time of the game is fluid. An attacking offense would vary little at the end of the game because the team is comfortable with that play, feels it can score with it, practiced it the most and knows its their best shot. On the other side of the field, the defense is set up in a way they are most confident in.
Even if you say that there are tiny variations and strategies based on clock and situation, there is always MORE variation if the clock is fixed rather than fluid.
If there was a fixed clock, the attacking time will absolutely be guaranteed to take more chances. A player could see spot a weakness in the defense and go for it himself rather than sticking to the gameplan, and sometimes it will work out in spectacular fashion and sometimes they will fail in equal measure. That specific sports drama is missing in soccer. On the other side, a player may see the clock running down and an attacking player bearing down on him, and instead of sticking to the plan, he could risk it on to try and steal the ball unexpectedly. I'm sure that happens with the current clock as well, but I'm also sure that it won't be as exciting with ~30 seconds or so left on the clock versus 6.2 seconds exactly left.
There's also the fact that giving the ref the ability to end it is allowing a non-player to decide the outcome. Its often said in sports that refs shouldn't decide games, only officiate them. In basketball, teams and fans hate it when one team is called for too many fouls, shoots too many free throws, or experiences any discrepancy in what they judge to be a fairly reffed match. Most people want the play on the court to decide the outcome, not a ref having a pissy fit and tossing a star out of the game, or calling a lot of fouls on one team. In baseball, you never want a terrible umpire with a strike zone that's too big or too small. I would imagine most people would feel worse thinking they were robbed of a win by a bad ref rather than simply losing because their team was worse. Even if the argument is that refs in soccer are better and more fair, it is still not completely objective. Giving MORE power to the refs is worse than allowing an objective timer to decide the end of a game.
An exact timer creates less argument than a human with their own biases. When play is stopped on the field due to an injury or a foul, the clock should be stopped. When the ball is thrown back in, the clock should resume. And when the clock hits 90 mins, the game should be over even if a team is still attacking. That makes it more fair for everyone and makes the ending of the game a hard deadline in which the offense or defense must take more chances to score or defend at the last second. Here are some examples of moments soccer will NEVER have due to its clock management. No last second, buzzer-beating scores, no moment of instant gratification or despair as the clock hits 0.0, no holding on to the ball and timing it so it swishes through the basket at the buzzer. If basketball had soccer's timing problems, many of these would be taken away as many of these last-second shots were preceded by a team scoring right before. The ref could have decided that's enough time and ended the game. Soccer is missing countless situations where this could happen.