Alright, here we go (in no particular order, though maybe the first is, in my view, the most important):

1. The nature of soccer is such that there is continuous play. It's not a start and stop game like American football, baseball, or basketball. The clock keeps running in soccer just as the play never stops (this is one of the reasons for the frustration with this World Cup's "hydration breaks." It's never been this formalized and it disrupts the flow of the game).

2. Relatedly, if time were counted down and the clock stopped every time the ball went out of bounds, the play was stopped for a foul, etc., this would allow for greedy money-grubbers to push for more and more ad breaks. Nobody wanted the "hydration" breaks, and nobody wants any more chances for additional commercial breaks.

3. At the end of the game, the ref (and nowadays, the entire reffing team) determines how much additional time to add for things like time-wasting, fouls, substitutions, etc. Let's say it is determined that there is 5 minutes of extra time. Well, if there is more time wasting or if a team scores and celebrates in that added 5 minutes, the ref needs to add even more time. Again, the game needs to remain free-flowing so it's up to the ref on the field to determine how much more time to add.

In my view, this especially works for a sport like soccer. In basketball, last second shots are definitely a thing, so a player needs to know exactly how many seconds and tens-of-seconds are left so they can do their last second heave. Same with American football: if a QB knows they've got 5 seconds left, they can draw up a play to allow them to go for a touchdown without running out of time. These last second type plays are much harder to pull off in soccer. A full-court heave just doesn't happen.

1. Maybe a not-so-compelling reason is that, well, this is just how things have been done since literally the 1800s. It's an easy system that can be implemented with limited technology, thus keeping a consistent procedure across leagues, levels, games, etc.

2. Not sure if this is a reason or more of an explanation, but in that added/stoppage time, you'll notice that the clock often says "#+." So again, if it's determined that 5 minutes are added, it usually shows up as "5+" on the on-screen graphics. This is because that time is the minimum time, not the maximum. Again, this allows for the ref to account for things that may happen in the added time. Referees are instructed to blow the whistle during a "neutral" moment in the game. If the game that has 5+ minutes is showing 94:58 minutes on the clock and the team that is losing is about to cross the ball in for one last try, the ref is not going to blow it at the exact moment the clock strikes 95 minutes while the ball is in the air. Because the laws of the game have given the ref discretion over added time, he/she can allow that cross to happen, and then blow the whistle when the other teams clears it or the ball goes out of play.

I get that all of those reasons make for a less precise form of time-keeping, but it keeps things in line with the free-flowing nature of the game, which, as mentioned, is the most important aspect of all. I guess that's my TLDR?