Captain Kidd is the only pirate known to have buried any of his loot, and not because he was saving for retirement.

Kidd was a successful captain and privateer when he was issued a letter of marque in 1695, ordering him to hunt for pirates in the Indian Ocean. After he set sail, he promptly angered the Royal Navy first by failing to salute one of its vessels, then by fleeing from a second ship that was attempting to - illegally - press some of Kidd's crew into the navy. Kidd compounded things by seizing the Quedagh Merchant, a supposedly neutral ship captained by an Englishman who promptly complained to the British government. (Sidenote: In the 20th century, it was discovered that the Quedagh Merchant was travelling under a French pass, which meant Kidd seized it legally.) To top it off, when Kidd finally did catch up with a pirate, instead of attacking him Kidd's crew testified that Kidd toasted the pirate's health and gave him presents! Other sources claim that Kidd did try to attack the pirate, but Kidd's crew mutinied, joined the pirate, and threatened to kill Kidd. Regardless of what really happened, Kidd's conduct angered the British government enough that, when they offered a general amnesty for pirates they specifically exempted Kidd from being eligible for it. It probably didn't help matters that Kidd was an outspoken Whig under a Tory government.

After learning he was screwed, Kidd buried some of his loot on Long Island and bribed the locals to keep silent about it; his plan was apparently to trade the location of the treasure for his life. After Kidd surrendered, his crew ratted him out and the authorities dug up the treasure, which was produced as evidence at Kidd's trial and Kidd was hung. To add insult to injury, the rope broke, so Kidd had to be hung a second time to finish him off.

Generally speaking, the life of a pirate was nasty, brutish, and short. What plunder they took was spent on ships' stores and whores, not buried for retirement. Privateers had investors they needed to pay back, and generally didn't need to conceal their treasure anyway, having come by it legally.