There could be lots of reasons for this
The exact reason why some house don't have Valyrian steel is not explicitly explained. But based on:
Valyrian steel was manufactured in the Valyrian Freehold with dragonfire before the freehold fell.
Valyrian steel was always costly, but it became considerably more so when there was no more Valyria, the secret of its making lost with the Doom.
Only the greatest weaponsmiths can reforge swords from existing Valyrian steel, making those remaining weapons highly treasured and extremely rare.
We can make following deductions from the above statment:
- Valyrian steel was costly.
- New Valyrian steel could not be created.
- Only the best weapon smiths can reforge swords from existing Valyrian steel.
At long last, Father? Valyrian steel blades were scarce and costly, yet thousands remained in the world, perhaps two hundred in the Seven Kingdoms alone. It had always irked his father that none belonged to House Lannister. The old Kings of the Rock had owned such a weapon, but the greatsword Brightroar had been lost when the second King Tommen carried it back to Valyria on his fool’s quest. He had never returned; nor had Uncle Gery, the youngest and most reckless of his father’s brothers, who had gone seeking after the lost sword some eight years past.
Thrice at least Lord Tywin had offered to buy Valyrian longswords from impoverished lesser houses, but his advances had always been firmly rebuffed. The little lordlings would gladly part with their daughters should a Lannister come asking, but they cherished their old family swords.
A Storm of Swords
- And house who have Valyrian steel, do not sell them.
So unless a house was lucky enough to have had a Valyrian steel as heirloom it is very difficult to get one. So lots of houses did not have them.