There are a few reasons for this. Some languages arent Context Free Grammars (the classic example for this is "A * B;" is this multiplication of the A and B var

There are a few reasons for this. Some languages arent Context Free Grammars (the classic example for this is "A * B;" is this multiplication of the A and B variables or is it a declaration of the B variable with the type being a pointer to an A) which make writing the grammar much harder in the first place. Some solutions to this are using a GLR parser instead, but that has the downside of super high execution time in the worst case. Lastly, even if you overcome all of these with the proper structure and hints, you run into pretty bad error cases. I.e. handing an invalid input is hard and giving an error to the user that makes sense is even more difficult. (which of the possible branches in the grammar rules is the one thats wrong? in simpler grammars you can probably give a good error, but a C grammar is going to have a much harder time) This is why gcc moved away from using a grammar to a custom parser IIRC.