MOSFET pins supports high currents?
Read the datasheet carefully. For example the IRF4104 has this:-

See that circled 6? That means note 6, which says:-
&quot;Calculated continuous current based on maximum allowable junction temperature. Package limitation current is 75A.&quot;

to handle currents of 50A, 6mm diameter cables are needed

Standard plastic insulated wire in a cable might be rated for 50 A (this site says 75 A for 'power transmission' and 158 A for 'chassis wiring') but MOSFET lead wires can safely run much hotter. Also the FET's rating is with appropriate cooling. The chip is rated for operation at 175 °C (so the package material must handle that) but with the case temperature held at 25 °C.
Bottom line is, ignore the absolute maximum ratings and keep the temperature down to a level that ensures reliable operation (eg 50°C). Depending on what cooling you have, that could be a lot less than 75 A. If you want low loss it might be lower again, eg. for 0.1 V drop the maximum current is only 25 A.