Why in Arc flash study the motors below 5HP are not considered? Is it there any standard for that?

It is 50hp (37kW) motors not 5hp. This is noted in IEEE 1584-2002 Clause 4.2 and larger than 50hp motors need to be considered as they can contribute to the total fault current on a bus.

Large motors (over 50 HP) are inductors which store energy in their magnetic fields. During a fault, they become generators for a short period of time and increase the fault current. Small motors are too small to have enough energy to make a substantial contribution so they are ignored.

Second consideration and this is more of a practical one is that the overcurrent protection for a small motor will limit the incident energy to an acceptable level. Generally this holds true for breakers that are 100 A or smaller where the cable is less than 200-300 feet long and the voltage is 480 V. But I know of no detailed, published scientific study or engineering standard stating this.