How do you define your "complex"? In your description above, I saw only sheer numbers of pieces composed. This is not a very good measure.
I suggest you first make yourself sound understanding to some fundamentals of music: timbre, beat and tempo, meter, pitch and mode, intervals and tunings, tonality, key signature, melody, texture and harmony, etc, etc.
Baroque music and classical music have their own characteristics. Generally speaking, in baroque music you do hear a lot of embellishment, highly constructed polyphony and alignment (counterpoint), which make it sounds "complex" at first hearing. But in terms of motivic development, rhythmic variety, emotional expressiveness, I'm afraid many works in the classical era are more complex.
Mozart, whose music many considered "simple" at first hearing, is considered the most complex and challenging to play among many top instrumentalists.
Some complexity is at the surface, you get it when you first hear it. Some complexity is in the flesh and bone, top instrumentalists practice it life long and always find new details and new meanings.
Of course, this is not to say, that baroque music is all that superficial.
Suggested learning: Robert Greenberg: How to Listen to and Understand Great Music, and Robert Greenberg: Fundamentals of Music.
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The question author commented:
I wasn't at all saying one was more complex than the next, I was just
trying to understand why if Baroque (which etymologically means
complex) is more complex, why did Vivaldi make more music than Mozart?
You made your question a bit clearer. OK. But still, how do you measure the amount of music Vivaldi and Mozart composed? Numbers of pieces alone is not that meaningful. Some pieces can be very short and some can be very long. I forget exactly in which piece (must be a baroque or classical one, may well be Mozart's work), a single movement has more than 500 bars, that means, more than 1000 beats. And even bars and beats are not good measures. Some pieces can be very dense of music notes some not so dense. So, please forget about these less meaningful comparison. Simply listen to their music and enjoy, and learn the fundamentals if you want to enjoy their music deeper and more.
Aside from the questionable measure about the music amount, generally speaking, baroque music has more clearly defined forms and structures. At one hand, this restricts the composers; At another hand, those rules and forms are somewhat like mathematical formulas, it is not that difficult to produce "stuff" by simply following those rules. Whether that "stuff" is good or not, is another thing. Within the same baroque set of rules, great talents like J.S. Bach, produced incredible master pieces, I may just produce some mechanical sounds.
While in classical music, humanism is the root, human emotion is more involved, bold and creative expression of composer's personality is often present. Composers in classical era often broke rules of baroque music. This can also mean more intellectual and emotional engagement while composing. Creating your own musical rules and styles could often be more exhausting than just following existing rules and styles.
Put it in another way, if you are a programmer, baroque music might be easier to imitate using a computer programme than classic music.