how come less and less skaters perform the scratch spin at the higher levels?

In high level competition, it's not about "looking amazing". It's about getting the highest score.

Under the IJS system, a scratch spin (counted as an Upright Spin level 1) is worth 1.20 points. When you compare it to, say, a Change foot Combination Spin level 4, which is worth 3.50, the idea of doing a plain upright spin is laughable. As the other answers said, it's basically throwing points away. It doesn't matter if it "adds heaps to a program".

You do see them perform scratch spins occasionally in exhibitions or artistic programs, but not commonly - I'd rather watch a difficult combination spin than a fast scratch spin any day.

A scratch spin is worth next to nothing compared to a combination spin or even just a basic camel or sit spin. A lot of skaters actually finish their program with a long scratch spin with their arms in some fancy position, but it's just not worth enough to leave it there by itself. Also, only 2-3 spins are allowed (depending on the level) and using a simple scratch spin instead of a more complex spin as one of them is basically throwing points away.

Other spins are worth more points, so its better for the skaters to do, say, a sit spin variation, instead of just a scratch spin. For me, I just don't really like scratch spins, I never have.

in artistic and fun, show programs people will do scratch spins, but when it comes to getting points, much higher combination spins are needed

I think scratch spins look really good if they are done well too but people don't often perform them in competition because they aren't worth as many points as other spins.