I'm wondering why isn't there any language agnostic AST, commonly referred to as intermediate representation, and higher level logic such as data flow graph built on Tree-sitter. I mean writing the Grammer is of course something that is mandatory but it seems like programming languages are too much different in the architectural level of Tree-sitter
Where we've found success is in stepping back and creating formalisms that are language-agnostic to begin with, and then using tree-sitter to manage the language-specific translations into that formalism. A good example is how we use stack graphs for precise code navigation [1], which uses a graph structure to encode a language's name binding semantics, and which we build up for several languages from tree-sitter parse trees [2].
[1] https://dcreager.net/talks/stack-graphs/
[2] https://github.com/github/stack-graphs/tree/main/languages
For your data flow graph, are you going to handle copy constructors, assignment operator overloads, implicit conversions etc. like you see in C++? Or how about overload resolution: figuring out which method wins out over all candidates requires details about the convertibility of types, and how the language ranks candidates based on conversions required. And let's not forget Koenig lookup.
Things like Tree-sitter can discover definitions, declarations and invocations, but matching the right declaration for an invocation isn't trivial in the presence of overloads.
Consider three languages, one with binary operators but no function application, and another with only function application, and a third with both. What would the "agnostic" AST be, except a union of the AST of the first two?