Access =/= Obtain
If players in an FFA game are equally good at clicking, the loot acquired per each character is the same between FFA and personal loot games for the same number of players within the limits of binomial variance. This assumes all players are collecting all gear drops. In a personal loot game, you can imagine that specific player may not pickup some loot. The loot is essentially lost. This highlights that functionally personal loot is harder in this regard and that it will be more difficult to gear up. In FFA, one man’s garbage is another man’s treasure so it makes it easier to gear up. Hopefully, this debunks in part some myths about personal loot.
You have also used monster kill drop as the metric. You have ignored that in single player a player does 1X damage and the monsters have 1X hp. In contrast in a 8 player game, the party can do 8X damage (and combat effectively monsters with diverse elemental immunities) while monster hp is 450%.
Therefore is it not per monster drop that is critical. It is monster drop per unit time that is the key consideration when comparing single and multiplayer games. For a game of specific number of multiplayers, total drops are constant. Therefore, the average number of drops per player is also constant between FFA and personal loot games (with binomial variance).