The +5 comes from assuming a 50% chance to hit. That means that you miss on 0.5*0.5=0.25 (or 25% of the time) and you therefor hit 75% of the time. This is equivalent to +5.
Now, you would be correct in pointing out that this is the maximum benefit possible from advantage. It declines as you both raise and lower your chance to hit. It does overestimate the value of advantage a bit.
However, it is almost certainly a better estimate than assuming an even distribution of hit chances. In a well balanced game you will not see very many situations where you hit on a 2, or very many situations where you only hit on a 19 or 20. Given the actual to-hit chances that you will encounter in BG3 the value is probably between +4 and +5.
