The reality is that although our Jedi heroes seem to lose their lightsabers with alarming regularity, most Jedi would keep a single blade for their entire adult lives.
As Obi-Wan makes clear in Attack of the Clones, the inability to keep hold of a saber is one of Anakin's key failings and something that has been drummed.into.him.repeatedly:
A familiar yellow speeder dropped to a resting place on the side of
the street. “Anakin!” Obi-Wan walked toward the young Jedi, pointedly
holding Anakin’s dropped lightsaber in his hand.
“She went into that club, Master!”
Obi-Wan patted his hand in the air to calm the Padawan, not even
registering Anakin’s surprising use of the feminine pronoun.
“Patience,” he said. “Use the Force, Anakin. Think.”
“He went in there to hide, not run,” Obi-Wan reasoned.
Obi-Wan held the lightsaber out toward his student. “Next time try not
to lose it.”
Obi-Wan pulled the precious weapon back as Anakin reached for it, and
held the young Padawan’s gaze with his own stern look. “A Jedi’s
lightsaber is his most precious possession.”
Once again Obi-Wan pulled it back, never letting Anakin go from his
scrutinizing stare. “He must keep it with him at all times.”
“I know, Master,” Anakin replied, a bit of exasperation creeping into
“This weapon is your life.”
“I’ve heard this lesson before.”
Obi-Wan held it out again, finally relinquishing that awful stare, and
Anakin took the weapon and replaced it on his belt.
As to why they don't carry spares, the answer is that Lightsabers take a considerable amount of effort to make, most Jedi only fight with one blade and most Jedi don't see enough action to justify carrying a pair of (largely ceremonial) blades since they almost never lose them. In short, the additional belt space could be used for something more useful like an aquata breather or a grappling hook.