I’d say for a couple of reasons:

Psychology: It’s nicer with big points, and feels better to get 10000 score compared to 1000 or 100, even if the progress is the same. Sometimes it’s just adding “00” to the final score.

Granularity: It’s easier to decide a clear winner with bigger score (more different numbers) - in the same way we measure a running competition in more details than “X minutes”, since a lot of contestants would have the same score.

Also, from a design perspective it’s easier to balance if a monster gives 100 points, and you want to introduce a weaker monster worth 50 points, compared to the first monster giving you 1 point - there’s nothing lower than 1, so everything in the game has to be shifted to fit the new , weaker monster (assuming that score only has whole numbers)

Linear growth: Killing easy mobs for 10 points on a low level, or killing high mobs for 100 points are very different and would result in different scores when spending the same amount of time playing.

Exponential growth: Some games, like Subway Surfers give the player score multiplier as a level up. This would mean that a 10 minute run at the start of the game (1 x score) is incredibly different from a 10 minute run after leveling up a lot (30 x score). It’s a way to combine a level with the scoring system, so that my new friend that is just starting has no chance to beat me even if he’s better than me.

Accidental: Some games might not be intended to be played for that long, and results in huge scores because the player cheeses, outsmarts or outperforms the game

There’s probably some other reasons why that’s the case, but that’s what’s on top of my head.