A main source of questions are those of beginning university/college students, or those at this level, which can include some advanced high-school students. More abstractly, persons towards the beginning of their tertiary education, or very close to the end of their secondary education.
Now, a main subject of study in mathematics at this level is calculus/introduction to real analysis (limits, differentiation, integration) and notions of linear algebra (solving systems of equation, vector spaces, linear independence, determinant). These subjects are obligatory for many students, not only those studying mathematics. By contrast, courses on elementary number theory are not nearly as universal in curricula in mathematics, and even more important are mostly confined to mathematics and closely related fields while courses in calculus are taken much more broadly. For example, many engineering and science programs will contain such courses.
Now, in a way this only moved the question to why the source I mentioned above is a main source.
That there are not more questions from more advanced students is explained on the one hand by the sheer numbers, especially as we lose most of the non-mathematics students, and on the other hand by the existence of a second site for mathematics questions (MathOverflow) that absorbs a lot of questions related to current research in mathematics and questions of more advanced students (students in the process of writing a doctoral thesis, mostly).
The question why there are not more questions from less advanced students is somewhat less clear. I believe that for average students at a younger age, in particular those not specifically interested in mathematics, the idea of searching information and help on the internet just might not really arise or it might just not be viable and efficient, as there are other more accessible sources for assistance.
You mention the International Mathematics Olympiad. Indeed, students involved in this and related competitions could be, and are, a source of questions. But, there are also other venues for this, for example, see https://artofproblemsolving.com/ and like various others I do not know about.