Because they were changing more than one aspect of humanity. They did things beyond spirituality. Like built cities, invented musical instruments, started revolutions, turned a conquered ppl into rulers, United many different types of ppl, and traveled many many countries and distances. Like Guru Tegh Bahadur focused on east India. From Nepal to UP (patna). Guru Gobind Singh the mountains area of anandpur and then Hazoor sahib. Guru ramdaas the Amritsar area.
Did u know each Guru built a city?
Kartarpur, khadoor, Goindwaal, Amritsar etc. They built water sources like wells and baolis, invited skilled workers called kalikaars to adorn and merchants to set up shop.
Taught sikhs kirtan, weaponry and warfare, how to read and write (invented an alphabet), changed culture( no sati, no evil, widows can remarry) created sikh rituals, wrote Gurbani, fought wars, traveled, taught sikhs the arts and literature (via calligraphy and dasam granth which has many translation of famous poems and stories of the time), etc
The Gurus did not just want to spread the message but actually wanted to implement it.