Why is it recommended that the parts to be glued be pressed firmly against each other?

The force of interaction of molecules increases, the smaller the distance between the molecules, the stronger they are attracted, thereby (pressing) the force of interaction and adhesion strength increase.
By themselves, the parts cannot connect, since the distances between the molecules on the surfaces are much larger than the sizes of the molecules themselves and the force of attraction is small. With the help of the force of pressure (with which we press the parts together), the layer of glue is thinned, reducing the likelihood of breaking the bond between the molecules.