I'm not too familiar with Japanese history but considering the widespread use of the katana which specialises in cutting flesh, why didn't they use plate armor like western knights? I thought samurai were relatively well paid and high in social hierarchy so if they had to pay for it, wouldn't it have been possible?
Because Japanese warfare, i.e. weapons and such, didn't 'force' the development of plate armour until contact with the West - there was no need to, and so plate armour simply didn't exist. When the matchlock was introduced, some armours were modified to incorporate the strong steel chestplate, whilst retaining traditional limb coverage. Also - the katana was never a common battlefield weapon, and only saw major popularity during the Edo period (when Japan was relatively peaceful). Until then, more common weapons included spears (yari) and bows combined with cavalry, and occasionally polearms.