why does tom invite Huck Finn to go treasure hunting ?

Huck is Tom's closest mate and they "adventure" together.

Also,

Throughout the novel, Tom and Huck pretend to be pirates and search for treasure: gold, silver, jewels, and every worldly good imaginable. In some ways, this hunt for treasure parallels the plot of the novel; the more the boys dig and search for treasure, the deeper they get themselves into trouble. Their quest for treasure is perhaps symbolic of man's quest for adventure in life. This meaning fits well into the theme of the frontier, which Twain reinforces with his romanticized sense of adventure.