println(fizzbuzz(i));
The honest answer: because it shouldn’t work and has never been done before. TeX has no arrays, no functions with return values, no convenient local variables or stack frames, no integers bigger than 2^{31}-1, no bitwise operations. Macro expansion can recurse, but you get no call stack and deep recursion quickly hits engine limits. What TeX does have is a Turing-complete macro expansion engine and, with e-TeX extensions (used by modern pdfTeX), up to 32,768 integer registers called \count. That turns out to be just barely enough to implement a chess engine.
圖像來源，費思兔文化娛樂股份有限公司，详情可参考新收录的资料
20+ curated newsletters。新收录的资料对此有专业解读
count = count + 1;
PRs: #11208 #11251，更多细节参见新收录的资料