The rules for D&D 5e clearly state that Extra Attack can only be used on the character's turn:
Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.
This on-your-turn restriction occasionally leads to scenarios in which creatures that roll a low initiative seem to have substantially better tactical positions in the turn order. The simplest example is when two level 5 fighters begin combat 60 feet apart. The fighter with the higher roll is faced with a choice: (1) dash up to the other fighter or (2) ready an attack action and (optionally) move closer. In case 1, the slower (lower initiative) fighter gets a full attack round before the faster fighter gets any attack. In case 2, the slower fighter has the option of moving to a stalemate position (i.e., where the faster fighter would be in an equivalent position on their next turn) or, if the faster fighter moved close enough, moving into range and attacking the faster fighter. At best, the faster fighter is going to attack the slower fighter once while the slower fighter will be able to use all of their extra attacks, not to mention a bonus action and reaction.
This example is contrived, but I've experienced versions of it in actual play. Simultaneously, I haven't been able to think up any situations in which allowing ready multiattack would be tactically unfair—this doesn't feel right to me, though. What am I missing?
On forums I've seen it conjectured that readied actions are disempowered to keep combat and initiative manageable, and this makes perfect sense to me, but I'm wondering if allowing readied multiattack would unbalance the game mechanics for a group and DM that enjoy tactical complexity.