Why does nearly every form of art depict Jesus as having long hair despite there being no mention of this in the Bible?

It's worth noting that the earliest surviving depictions of Jesus show much more variability than what becomes the "norm" later on. These are portrayals from the 3rd century (I don't know if there is anything earlier than 3rd century).

The Santa Maria Antiqua Sarcophagus, for example, from about 270, shows a short-haired, beardless "Good Shepherd" who we can read as a Jesus/Christ figure (images)

Similarly, there is a Good Shepherd fresco from the 3rd or 4th century in the Catacomb of Domitilla, Rome, Italy, that can be read as a portrayal of Christ (perhaps in the guise of Orpheus). Again, the figure is beardless with short hair.

Christ could also be shown as a sun-god (or, more accurately, artists could appropriate sun-god imagery for him), as in the Helios (Christ as the Sun God) mosaic in the vault of the tomb of the Julii, in the necropolis under St Peter's Basilica (late 3rd century).

There are other examples, but these should illustrate the point that early on there was variation and, if anything, short-haired and beardless was the norm.

We can see an early example of the bearded, long-haired Christ in the apse mosaic in Santa Pudenziana, Rome, c. 400. Here Jesus is older, more in line with a wise teacher or scholar, seated on the throne in the Apocalypse. A similar long-haired, bearded Jesus is seen in the crucifixion scene (the earliest to survive?) on the wooden doors of Santa Sabina in Rome, c. 422. Of course, Jesus's image is still fluid at this time, and we can easily see him as a teacher but still as a young man (beardless, short hair), as in apse mosaic in the Chapel of Sant’Aquilino, S. Lorenzo Maggiore, Milan, ca. 400.

By about the sixth century, however, the bearded, longer haired Christ is the norm, seen in images like the icon of Christ and St Menas as well as in icons like the Christ Pantocrater.

The shift probably has something to do with the shift in how Jesus was conceptualized by worshippers: by the sixth century, he looks less like the gentle shepherd and has become a more powerful, Zeus-like figure. A change in iconography would have been appropriate given the change in perception.

It's worth noting that there is a letter which purports to describe Jesus's appearance, although this is from much later.

If you're really interested in learning more, the seminal work on Jesus's appearance in imagery is Ernst von Dobschütz’s Christusbilder: Untersuchungen zur christlichen Legende (Leipzig: J. C. Hinrichs’sche Buchhandlung, 1899), in which the author addressed issues about the legends, history, and cult surrounding the "Holy Face". More recently, check out Herbert L. Kessler and Gerhard Wolf, eds., The Holy Face and the Paradox of Representation, Villa Spelman Colloquia, vol. 6 (Bologna: Nuova Alfa Editoriale, 1998) and Hans Belting, Likeness and Presence: A History of the Image before the Era of Art, trans. Edmund Jephcott (Chicago: University of Chicago Press, 1994), especially chapter 11, “The ‘Holy Face’: Legends and Images in Competition.”

EDIT: it's also worth remembering that many things that people now accept about Jesus had to be established or agreed upon well after the fact (assuming you believe in a historical Jebus). Things like his divinity and his full humanity, for example, were argued about in the early Church (and later). Despite what people may think or believe, the Bible as we know it is silent on many aspects of Jesus's life and status that believers and nonbelievers assume is gospel (pun intended).