how come i get no experience points for killing goblins when i started over with my previous characters who are at 15th level and beyond

It's because of 3rd edition rules. Goblins are not a challenge for 15th level characters so killing them give you no experience.

Because you're 15th level and beyond, that's meant for Heart of Fury mode. Offcourse you get no xp for sneezing and opening your eyes to see that the booger-projectiles you launched have just about killed every goblin in a mile radius. If you want you can enable HoF mode in the Configuration program.