We may read what Whitaker has related in his "A Complete System of Universal History" (1821) about Jedso (Hokkaido):
Their weapons are bows, arrows, and lances, and a kind of short
scymetar. On some occasions they use poisoned arrows, being choleric,
quarrelsome, and revengefull. Instead of shield or cuirass, they wear
coats made of small, thin laths.
Charlotte Salway, a famous sinologist, wrote:
The mighty sword and arrows were the chief defensive weapons. These
they made in endless forms and fashions to suit all requirements—for
distant practice or hand-to-hand fighting. Generals carried flat and
closed fans, for giving direction on the battle-field or for use as a
shield at close quarters.
In general, there were several factors which discourage the use of shields. Firstly, metal was relatively more expensive in Japan than medieval Europe. Therefore, what metal they had tended to be worked to higher quality and used in offensive weapons. It would have been seen as wasteful to the Japanese to use metal on a shield, since it could be made into something much more valuable: a weapon.
A second factor derives from the first. Since the Japanese lavished more attention on their steel it tended to be higher quality. Good quality steel can cut through any typical shield. A shield that could withstand a well made sword would be very heavy and expensive, completely ineconomical. Early knights used shields because they often fought peasants who used clubs and rocks.
The third factor is philosophical. Japan from 1608 was in the great peace of the Shogunate, so practical war technology was secondary to politics and etiquette. This led to a sword-centered philosophy. Salway writes:
Arms were in great request during the turbulent times, when the
Minamoto and Taira clans contended for the right of governing the
people in a military sense; but when Ieyasu settled the long dispute
and restored tranquillity, the beautifying of swords and sword
furniture became a matter of great interest to all privileged to carry
these protective weapons. In the sword was centred all the pride of
the wearer; it became his dearest friend, the guardian of his honour
as well as his greatest treasure.... Upon the sword, as upon
everything else which these Orientals produce, labour was abundantly
lavished; every portion received minute attention—-the guard, the
blade, the hilt, the scabbard, down to the smallest accessories. Not
only was it an appendage of dress in the daytime, but it was carefully
laid aside in a suitable resting-place at night; and in the
living-room of every house was seen a raised dais, with a stand
specially provided for the purpose of holding the sword.
A samurai would have considered use of a shield, even when facing arrows or stones, as a dishonor because it would seem cowardly or self-serving; in other words a shield is inconsistent with the principles of bushido.
Also, just to make the point concerning the quality of Japanese steel, even today it is the best. For example, the best shears in the world are all from Japan. Here is an example from Naruto, one of the better makers:
That's right, $2000 for a pair of scissors. And yes, they cut through medieval European shields like butter.