It takes 2k xp for a fighter to get to level 2. If you take a common enemy at that level, a hobgoblin worth 35 xp, that fighter would have to kill 58 of them. Now let’s average that out for a generic party of 6: 330 hobgoblins to kill for them all to get to level 2. At an average encounter size of ~6 hobs, that’s 55 battles. Even if you mix that up with the other low-level creatures like gibberlings, bandits, and kobolds, those things are really only going to increase the number of trash battles. Where the most xp gains come from are “bosses” and quests, the latter sometimes worth more than all of the battles leading up to their conclusion. Even the simplest of fetch quests are considerably more profitable xp-wise than combat.
What combat provides, generally, is loot. Considering that any particular build of character is only going to progress through, at most, three iterations per equipment slot before achieving end-game gear (for BG1 and BG2 taken separately), the vast majority of loot is either kept as trophies or, more likely, sold in order to purchase consumables or gear unavailable through combat. In other words, the essence of combat in BG is nothing more than violent haggling.