Why do I still have seizures even thought I had surgery and had my brain tumor removed?

Not uncommon. Unfortunately, even with full tumor resection, only 80% of patients will have complete seizure disorder relief. 10% or so will note improved seizure frequency/intensity and the rest of the patients will have no improvement of their seizures despite surgery.
Scar focus. Tumors do not cause seizures. They irritate regions of the brain that then cause seizures. After removal of a tumor the irritated brain areas may remain. In some cases further surgery is necessary to remove these areas. Eeg recordings during surgery can identify these areas for removal. Otherwise medications are appropriate to control the residual seizures.

Related Questions

I have a brain tumor that sometimes causes seizures from the scar tissue from my 3 surgerys & I wanted to know if you think Ill be on the meds forever?

Forever is long but- With that type of history of 3 surgeries and the formation of scar tissue I believe the only way to keep you completely safe from seizures due to the scar tissue irritating your brain is to keep you on medication. I know that's a long time. Maybe technology will change in your lifetime for this....not likely in mine though...PMA (Positive Mental Attitude)