Where do they “exist”? What does the word “exist” mean in this context? There are some philosophers who assert the “existence” of so-called “abstract objects”, and they say that these “objects” are not “created”, rather they are “discovered”. So, contemplate it. Numbers are merely a subset of these abstract objects. Do you really think that Hamlet was “discovered” and not created by Shakespeare? That every translation of Hamlet is a newly “discovered” abstract object? Or every edition with a misspelled word in it reflects a brand new “discovery” of a brand new abstract object? Do you really think that the rules of the game called “chess” or “bridge” were actually discovered, and not simply “made up”?
Yes. There was no concept of “pi” before the ratio of circumference vs. diameter was thought of. Mind you, if there would be a “circle” and its “diameter” in objective reality, their ratio would be approximating the value of “pi”. But numbers, circles, lines, points (zero dimensional dots) do not “exist”, except as concepts. What about the square root of minus one (usually designated by the letter “i”)? Or multi-dimensional spaces? We literally “make them up” because they are fun and useful.
Existence can be subdivided into physical existence and conceptual existence. Concept only exist as “imaginary” entities. If all the humans and their creations would disappear in a cataclysm, all our creations; literature, music, mathematics… etc. would simply cease to exist.
If some hypothetical intelligence could emerge somewhere in the universe, they could create their own mathematics and geometry, which might or might not be similar to ours, but the other “abstract objects” like Hamlet, or the Ninth Symphony would be lost forever.