Pros and cons of using resistor networks over individual resistors?
Typical advantages of using resistor networks:


Smaller part count, as mentioned. Therefore faster assembly.
Better matching of individual resistor values and drift, by being in the same package.
Saving board space: Not always, but when you account for the keepout area for individual parts, it's an even bet.
Cost of BOM: Not necessarily, but for sufficient volume, almost certainly.


Why every design does not use resistor networks:


Availability - Not as ubiquitous as individual resistor values
Coupling between resistors: Especially a risk with the smaller network packages
Cost: For smaller volumes, likely to be more expensive than the individual resistors
Layout convenience: It can sometimes get exasperating to fit traces around a 8-lane highway toll bridge of a network.
Flexibility: All the resistors in a network are typically the same value (R-2R networks do exist), while actual designs do not always need 4 or 8 resistors of the same value at the same location on the board.
As @ThePhoton mentions in his answer: Tolerances available