Why does instant coffee need more milk/sugar to get desired taste? This morning I brewed some coffee at work using a pour-over and noticed that the amount of milk and sugar required to get the desired taste and whiteness was significantly less. 

I do not mean because it tastes better it needs less, the amount of milk and sugar needed was less. Is this due to the density of instant coffee which requires it to have more for the taste to be recognizable? Or is there more to it?
This question is fairly ambiguous as you've simply described the result as a 'desired taste' so I cannot tackle the question from a specific angle. However, the differences you describe may be a symptom of the fact that there are big differences between instant coffee and your pour-over.

Actual coffee. Your pour-over must be done with coffee grounds and so you are producing a cup of coffee freshly extracted from the grounds and subsequently freshly enjoyed. Not to mention the quality of the beans used might simply be of a higher quality than that used in instant coffee manufacturing. 

Pour-over. The brewing method itself is often known to produce much more nuanced flavours as it allows the brewer control of water flow as opposed to other automated brewers. This means that you can better extract the sugars and compounds that give coffee its aroma and flavour. Instant coffee is not fresh and the general manufacturing process is done en masse so the level of control that you would have to fine tune your cup of pour-over coffee will simply not be present in instant coffee. Also, due to the extra processes involved in freeze-drying and packaging the instant coffee, many flavourful compounds can be lost well before it reaches your cup.

The factors noted above can change the resulting cup you enjoy and therefore change the amount of milk or sugar you will need in order for you to achieve your desired taste. Since the pour-over method can extract more sugars and delicate compounds that make coffee so delicious, it will also reduce the requirement of extra sugar.