Why is it so hard to do anything when You have the knight pair and your opponent has the bishop pair

I don't know why is playing 2 knights so hard against the 2 bishops that can do many things.

Depends on the position I'd say. However, we cannot ignore 2 Bishops can force a mate whereas 2 Knights fail to do so... A Bishop pair is naturally more powerful compared to a Knight pair

Have a look at this study. Its quite illuminating on that topic i think.

Depends on the scenario. If the position is closed, (ie there are very few pawn exchanges), then the knight pair will triumph over the bishop pair. This is because bishops struggle a lot in closed positions, since they will be "biting" at rock solid pawn chains, whereas knights can jump over pawns if needed, so it isn't an issue for them.

On the other hand, when the position is opened and there are lots of pawn exchanges, the bishop pair increases in strength, as they are long range pieces, and each pawn off the board maximizes their potential. Knights need outposts and stability, otherwise they become quite clumsy pieces, which is why you might be struggling in those particular positions.

The key is identifiying what kind of position you are playing (closed or open), and exchange pieces accordingly so that you are left with the more powerful pieces

Actually in the opening when you have an advantage in development (Nimzo Indian, Ruy Lopez Exchange, Rossolimo e.g.) you have to open the position as quickly as possible to launch an assault with the two Knights. Exactly the opposite what you do in the long run.

There are no general rules, it depends.