Page 168 of Gygax's DMG says "The various items listed generally have their bonus applied to both "to hit" and damage dice". Arrows aren't mentioned as any sort of exception, and the entry for Bow +1 says that it "gives + 1 "to hit" and + 1 damage potential to arrows fired from it. If magic arrows are used, the bonus is the total of the bow's and the arrows'."Remember the days of an arrow being shot from a magic bow was not a magic weapon for overcoming magic resistance?
In other words, magic arrows are stronger than normal magic weapons because they can be stacked. They're not weaker.
Page B48 of Moldvay Basic says "When a magic weapon is used, the user adds the magical adjustment to both "to hit" and damage rolls." Arrows aren't called out as any sort of departure from this rule.
The first heading on page B20 of Moldvay Basic is "Encubrance (Optional)".This was part of the game many editions ago, in the pen and paper days. Encumbrance wasn't optional, so you tracked everything.
<snip>
It just became a habit for the older players.
So for plenty of "older" players, it was optional.