Why is it difficult to drink juice from a straw or bag without letting air into it?

In this case, Bernoulli’s law will work. It establishes the following relationship: the fluid flow rate will be higher in the direction where the pressure is lower.
This suggests that in the absence of air in the tube or bag, the pressure in this place will be zero. And in view of this, pressure from the outside will act on the tube or bag, as a result of which the liquid will move at a higher speed towards the lowest pressure.
In the case of air, the pressure generated from the outside and from the inside of the tube or bag will be equal and the liquid will not drain inside.