A fair question!
You’re right: it is often just taken for granted that intervals are the starting point for ear training. And that’s unfortunate, because for many people (I suspect you’re one) there are other things which can usefully be learned first, such as chords, progressions, audio effects, etc.
Intervals are often ear training 101 because in terms of music theory they’re the simplest thing to train your ear for: just two notes. And they can be very useful indeed, and they do build a strong foundation for other things such as chords, scales, progressions, and so on.
Here’s a full post on the subject: “Interval ear training: what’s the point?”
If you’re getting on well with your current ear training and it’s closely tied to what you’re working on in music, stick with it! Don’t assume that because lots of other people are studying intervals you should too. That said, if you’re looking to add a new topic or build a really robust sense of relative pitch, interval ear training is probably worth some time.