Why don't the people in Laketown just relocate?

From the LotR wiki:

Uniquely, of all of the towns, settlements, fortresses, and cities of Middle-earth, Lake-town utilized water as its defense. The Long Lake was also surrounded by towering cliffs and high mountains, all helpful natural barriers that had the potential to aid its defenders in a siege (such as the Easterling invasion of the North in TA 3018).

So in other words, until Smaug the town had provided a very good defence against almost all forms of attack. The townsfolk were't living in fear of invasion or under any real threat and so felt secure.

It is worth nothing the following from the wiki as well:

...Smaug, livid with bestial fury, had winged his way to the town in the dead of night and promptly began to immolate the wooden matchwood structures there, materials that burst into flame at the slightest provocation. Its inhabitants realized all too late their predicament and while some, including most of the women and children, got away safely on boats, many perished in the maelstrom of fire that ensued. A brave company of spearmen and archers attempted, in vain, to shoot down the dragon, but were mostly immolated themselves. The town was wrecked by the dragon...

In other words, once they realised their predicament, many tried to leave the place to survive.

Finally, with regards to the rebuilding of the town after its decimation by Smaug:

The inhabitants also learned a crucially important, but painful, lesson bought with the blood of loved ones: the town was built to be less susceptible to flame and its military was expanded. These lessons greatly aided it during the War of the Ring.

This last step meant they didn't need to worry about relocation again after the death of Smaug.

Additionally, @invalid_iq's answer provides some excellent economic reasons why the townsfolk remained.

Most of them make a living of fishing and doing business with the elves. Relocating makes fishing impossible and also, most likely, stops them from doing business with the elves (which they do using the water).

So if they move to another location, they lose all there income and hence, they have to start from complete scratch. They think they have a pleasant life at the time being, knowing that the dragon wont wake up without a good reason. There's no point for Smaug to destroy the city, unless he can use it as a threat to get what he wants.

Compare with real life where people live in places that is plagued with regular disaster such as Bangladesh (flooding) or parts of USA (tornadoes). After they have lost everything the survivors just rebuild.