This association goes back to the iconography of St George.

While the story of the spear and the dragon was not part of the earliest hagiographies of St George (3rd-5th century), just a century later, images of a mounted St George slaying a dragon with a spear emerged in the South Caucasus , including the reliefs on the Brdadzori and Xožori stele in Georgia .

The earliest written version of the story of St George and the dragon associated also comes from Georgia and the 11th century. That first appearance has been carefully studied and translated as recently as 2020. It doesn't however mention a spear.

This telling of the story spread from the Caucasus throughout Europe with the Crusades, until it was captured and enormously popularised by Jacobus De Voragine (Jacob Of Voragine) in his Legenda Aurea (Golden Legend) written in 1265:

"Thus as they spake together the dragon appeared and came running to them, and St. George was upon his horse, and drew out his sword and garnished him with the sign of the cross, and rode hardily against the dragon which came towards him, and smote him with his spear and hurt him sore and threw him to the ground."

From here on out, almost all the iconography of St George, depicts him with a spear killing a dragon including in works by the greatest Renaissance artists and later grand masters. This iconography became absorbed into wider literary and artistic culture as a secular trope, and eventually landed fully in popular culture from comics to videogames and movies.

A good popular introduction to the imagery is this one. A much richer overview may be found in the first chapter of the fantastic Knights, Kings and Dragons: The Symbolic Conquest of the Mediterranean Sea in Late Antiquity and Its Historical Background by Joanna Töyräänvuori.

Christian iconography inspired by St George usually features a spear or lance, but the canon of famous Western dragon slayings offers a wider range of techniques and weapons. Here are a few:

Beowulf vs the Dragon - Beowulf uses sword and shield.

Sigurd vs Fafnir - Sigurd uses a magic sword and a camouflaged trench/foxhole.

Sir John Lambton vs the Lambton Worm - Sir John uses his sword and fights in a river so the bits will be carried away by the current and won't be able to join back up again magically (something that had stymied previous heroes' attempts).

Sir John Conyers vs the Sockburn Worm - Sir John uses his sword*

Thor vs the Midgard Serpent - Thor uses his hammer Mjolnir.

Heracles vs the Hydra - in different versions Heracles uses either his club, a sickle, or a sword. Heracles' squire Iolaus sometimes assists using fire to cauterise the neck stumps to stop the heads regrowing.

• Sir John's alleged sword, known as the Conyers Falchion, can be seen in the museum at Durham Cathedral. By tradition, it is presented to every new bishop of Durham, for use in case of monster attacks in the diocese. The sword is a late mediaeval falchion, a single edged chopping weapon. It is very large, and totally looks the part..