Warren Buffett has never done a stock split of Berkshire Hathaway Class A shares (BRK-A), and he has flatly stated that Class A shares will never undergo a split. Buffett's reasoning for not doing a stock split of BRK.A is right in line with his basic investment philosophy.
Buffett's investment approach has always been that of a buy-and-hold investor focused on value and long-term growth – the polar opposite of an intraday trader. In line with this fundamental approach to investing, he believes that allowing the price of Berkshire Hathaway Class A shares to remain at a level that encourages purchasing the stock for the long term, rather than trading in and out of it, attracts the same type of investor as himself – that is, investors with an extended investment horizon and investing strategies.
Another potential reason is that the Oracle of Omaha derives some satisfaction from Berkshire Hathaway being, far and away, the most expensive stock in the world.
Buffett created Berkshire Hathaway Class B shares (BRK-B), which sell for a small fraction of the price of Class A shares, with the stated purpose of enabling retail investors to buy Berkshire Hathaway stock directly. Berkshire Hathaway did a split of the Class B shares in 2010, and not at the traditional two to one or three to one rate, but at a rate of 50 to one. While some might argue that this action is contradictory to Buffett's stated no-split policy on Class A shares, it is in fact logically right in line with his rationale for the creation of the Class B shares – to make (and by doing the split, to keep) Berkshire Hathaway stock affordable to smaller investors.