Why does spoken Tamil drop so many sounds compared to what I read in my textbook?

I’m a grammar-focused learner trying to understand why 'enakkuttheriyum' (I know) sounds like 'enakkuttherla' or 'teriyala' in daily life. I want to learn the formal grammar, but I’m struggling to map those case suffixes onto the rapid-fire colloquial speech I hear in Tamil Nadu. How can I bridge the gap between textbook formal Tamil and the actual reality of street-level speech?