Tell that to a set theorist.
I talk about this to mathematicians all of the time. It is well known that mathematics is axiomatic at the foundation. This is something you learn in a first year algebra course when you are pounded with definitions, lemmas, corollaries, and theorems. Most people take this for granted, because they use base 10 and 12 mathematics every day, and figure that it must be based on concreteness - since it physically applies to our life, and we can "see" the math working and making sense.
Those sets - or mathematical objects - are by definition abstract objects. So, a set theorist should understand what I am saying from their own intellectual foundation.
All math comes from axiomatic assertions that create a foundation for us to apply meaning to abstract and arbitrary objects we choose to represent something. This goes for all disciplines; it is all arbitrary and abstract.
The number "1" is an element that comes from a set of objects - that are arbitrary. There is no such thing as a ring; we define what a ring is, and appropriate it to the mathematics. There is no such thing as "1," we appropriate that symbol as an element of the real integers, for example: "1"
is not a fundamental object of nature; you won't find it in nature. What you perceive in nature is its quantatative representation in nature based on the axiomatic definition we have made of the symbol to
explain facets of nature.
Sets are abstract and arbitrary; there is no such thing as a set,
as it were mathematically, in nature. We have defined the axiomatic foundations of a set such that we can it in "concrete" life.