Because those tracking scripts are provided by competing advertising platforms and they want to own the data.
Only if you show the ads. Many companies do not, but still use lots of trackers. Why? Because their performance marketing team is trying to find the right mix of advertisement channels, so they go for paid search and clicks to Google, Meta and lots of other AdTech. In that case trackers are needed to optimize spending by analyzing user behavior. If certain cohort spends more time on the site, they will get more ads of it. If another cohort leaves the site quickly, they will see less ads. The promise of AdTech in general is that they personalize ads as much as possible to reduce your customer acquisition costs (CACs) - you won't waste money on showing ads to people who won't buy your product. So they need the data and they have to own it, because personalization is their competitive advantage.
So they make it harder or more limited to integrate with tools like Segment.
Let's say tracking for revenue is required and not an argument to be made. The question I never quite understand is why cant we have ONE scripts to rule them all? I remember there was a company / services that may be called Segment? And quick google search doesn't have anything familiar, that offers something like that.