A fluid is in fact a collection of molecules which have certain mobility. You can think of a fluid as millions of very tiny balls, indeed so tiny, that they can move around in space, unaffected by gravity (this is not exactly true, but for these purposes, let's just assume it is). These molecules bounce with each other and with the container walls, and just like a ball bouncing on your body pushes you back, these molecules exert such push in their containers.

The explanation for your question lies in the 'Kinetic theory of fluids'.

All fluids are, of course, made up of molecules. At any temperature above absolute zero (0 K or -273.16 C), these molecules have a kinetic energy that is proportional to the temperature. The kinetic energy results in random motion of the molecules, which undergo collisions with each other and with the walls of a container. These collisions are the cause of fluid 'pressure'. It is immediately obvious that fluid pressure must be 'isotropic' - it is the same in all directions - and that as the wall area increases, so will the pressure force,

i.e. Force = Pressure * Area