To get an answer to the excellent question you raise, one has to go back approx. 250 years to a remote desert corner of a territory of the Ottoman Empire called Arabia. There, around the middle of the 18th century, an influential tribal leader named Sheikh Mohammed Bin Abdul Wahhab formulated a unique view on contemporary Islam, which saw the Ottoman sultans as corrupt, Westernized backsliders who were moving away from the purity of Mohammed's teachings.
Initially, his teachings were seen as heresy, rejected and even banned by the Ottomans, but his influence grew over time. The sect he founded came to be known after him as Wahhabism. Its early manifestation was of a terrorist nature, attacking innocent Muslim pilgrims and other travellers to Arabia. Fortunately, the Ottoman Empire, even in its decrepitude, was able to stop this menace from spreading outside Arabia.
But his teachings attracted numerous followers inside Arabia, and with the eventual ascension of the al-Saud dynasty in post WWI Arabia, and the subsequent discovery of oil, Wahhabism had large resources at its disposal.
OBL is simply following the philosophy of Abdul Wahhab, which abhors modernity and Western influence, which it regards as corrupting, and pines for a return to the early days of Islam, when brutal conquest helped spread the word of Mohammed.