Cyber-Dave
Explorer
Because, Nietzsche. 

Because characters are intended to work with synergy as a group rather than just several 1-on-1 encounters. Deciding what would constitute a 'match' for a single character depends so much on character build decisions that its not really do-able. Matching for a group, which is assumed to have varied capability and work together is much easier.Does anyone know why the decision was made to design CR the way it is? Why is a CR X monster intended to be suitable for a party of level X adventurers? If I had been designing CR, I would have built it so that one CR X monster was a match for a single level X character, and a party of level X adventurers would be matched against a party of CR X monsters.
This, and it is easier for it to become clear to the DM using CR to realize that it is intended as a rough gauge, not a precision measurement, when it isn't presented as being granular like a 1-on-1 measurement would appear to be.Because characters are intended to work with synergy as a group rather than just several 1-on-1 encounters. Deciding what would constitute a 'match' for a single character depends so much on character build decisions that its not really do-able. Matching for a group, which is assumed to have varied capability and work together is much easier.
Which makes sense, since fighting a clone of the party would result in a 50/50 outcome decided solely by the dice assuming the same level of player skill on both sides.
A deadly encounter in 5e is one that can be deadly for the party due to poor decision making or poor dice rolls, but that should always be beaten with average rolls and tactics equal to the DM.