That doesn't mean there aren't reasons for attempting this sort of thing, however. The enemy could die. Or your attack could have a rider that will prevent them from fleeing, like knocking them prone, pushing them over a cliff, what have you.
Can I cast a readied magic missile to use when the target begins to turn invisible? Why or why not?
If you’re unsure when a reaction occurs in relation to its trigger, here’s the rule: the reaction happens after its trigger completes, unless the description of the reaction explicitly says otherwise.
You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range.
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction.
For readying a spell or other action, does the target have to be in range? Your target must be within range when you take a readied action, not when you first ready it.
Player: "I want to ready my magic missile spell to nail him if he turns invisible. Can I do that?"
Taken that you select a spell's target(s) when you take your readied action and release the spell, which take place after the triggering spellcasting occur, any readied spell targeting creature you can see will only be able to target those you can see at this moment. If the caster is now invisible, it won't be a valid target for Magic Missile.
In this case, the casting of the magic missle was mostly completed on that player's turn and held, per the ready action rules
1) Be watching for the enemy to start to become invisible, regardless of the cause. (The trigger is "start to become invisible," although perhaps "start to disappear" might be better?)
2) Interrupt with the reaction to release the spell.