Why MOSFET source is indicated with arrow ?
IC MOSFETs aren't the same as their discrete counterparts

You are correct in that laterally diffused four-terminal MOSFETs (such as those that make up CMOS ICs) are symmetric devices -- the substrate or well is connected separately to the lowest or highest (depending on what kind of FET you have) potential in the circuit, while the source can be raised up above/lowered below the substrate/well potential.

However, 99% of the discrete MOSFETs made throughout history and 100% of the discrete MOSFETs in current production use a different structure -- instead of having the source and drain side-by-side, the drain is on the bottom and the source is on top, with the gate cut into the FET.  This is called a vertical MOSFET, and is depicted below in its modern form (i.e. a trench MOS structure -- early vertical MOSFETs used a V-groove for the gate instead of the trench).  These structures are inherently asymmetric, and also lend themselves to connecting to the substrate to the source, thus forming the body diode that is a surprisingly useful part of a power MOS device.