Why is Berkshire Hathaway so expensive???

To what I think, its because of investor's psychology. The reason is that an investor is more likely to invest in a "cheaper" stock in terms the market price because he can invest in more quantities of shares.

To prevent the stock from experiencing bubbles and ultimately a crash, BH has not done a stock split. The high price prevents investors with little to spare to speculate on the stock. It also prevents the stock from being overvalued as the price is already so high.

Investors generally will be more comfortable owning 50000 shares at $1 than 1 share at $50000.