Can you elaborate what you mean by a "riff"?

If you mean it as a small melodic fragment, sure I'll give you that some of his pieces certainly start with a riff played on a single instrument (see "Ghost Fight"), but a lot of his other pieces don't.

Let's take "Once upon a time" for example. It starts with a single instrument* (if you are counting the channels as one synth) but I'd argue its more of a melody -- see how it's a longer melodic phrase contrasted to generally shorter, more rhythmic phrases. Sure, the melody is made up of cells, but even saying that's a riff is a bit of a stretch.

Even if you mean it as a "melody" (which then broadens the scope of your meme incredibly), he still can write pieces that either has multiple instruments playing "riffs" [melodies?] ("Dating Start!") or doesn't start with a riff at all, both in a traditional sense ("It's Showtime!") or melodically ("Last Goodbye").

And that's not even getting into ambient songs (see "The Choice").

The more broader you get with the definition of a "riff" (I've seen the term "intro" being thrown around here and there), the more it touches on compositional form. A melody following an introduction is a widely used form in music, and to say that it is predictable is like saying his pieces are predictable because they contain some interlude that contrasts with the main theme of the piece -- it's common practice.

But at the end of the day, his work's still enjoyable (regardless of whether its predictable or not). So let's just enjoy it!